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1. End‐plate potentials (e.p.p.s) were recorded from frog sartorius neuromuscular junctions under conditions of decreased transmitter release to study the effect of repetitive stimulation on augmentation and potentiation of transmitter release. 2. The magnitudes and time constants of decay of augmentation and potentiation were determined both following a primary conditioning train and following an identical secondary conditioning train applied from 30 to 170 sec after the primary conditioning train. 3. The magnitude of augmentation following the secondary conditioning trains was increased over that following the primary conditioning trains even though augmentation, with a time constant of decay of about 7 sec, had decayed to insignificant levels before the onset of the secondary trains. This increase in augmentation was not due to a change in its rate of decay during the secondary trains. 4. The increased magnitude of augmentation can be described as arising from an expression factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1‐6 +/‐ 1‐2 (S.D. of observation) (the magnitude of augmentation is increased 2‐6 times) and decays approximately exponentially with a time constant of 90 +/‐ 50 (S.D. of observation) sec. The expression factor thus decays about ten times slower than augmentation. 5. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 2‐8 +/‐ 1‐0 (S.D. of observation) times increase in the magnitude of the expression factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 6. The expression factor, while increasing the magnitude of augmentation, had little or no effect on the magnitude of potentiation or on trasmitter release in the absence of augmentation. The expression factor decayed about twice as slowly as potentiation. 7. The time constants characterizing the decay of potentiation were greater following the secondary conditioning trains than following the primary conditioning trains. 8. The increased time constant for the decay of potentiation can be described as arising from a time constant factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1‐2 +/‐ 0‐7 (S.D. of observation) (the time constant of potentiation is increased 2‐2 times) and decays approximately exponentially with a time constant of 130 +/‐ 45 (S.D. of observation) sec. The time constant factor decayed about three times slower than potentiation. 9. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 1‐6 +/‐ 0‐8 (S.D. of observation) times increase in the magnitude of the time constant factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 10...

1. End‐plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation. 2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 sec (range 4‐10 sec) which is intermediate in duration between the time constants for the decay of facilitation and potentiation. 3 . The magnitude of agumentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0‐3 to 0‐6 following 50 impulses at 20/sec to 0‐5‐7‐8 following 600 impulses at 20/sec. (An augmentation of 0‐3 and 7‐8 would increase e.p.p. amplitudes 1‐3 and 8‐8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased. 5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal. 6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3‐8. The corresponding Q10 for the decay of potentiation was found to be about 2‐4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.

1. End‐plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions under conditions of low quantal content to study the long‐term effects of repeated synaptic activity on transmitter release. 2. The nerve terminal was presented with 30‐100 successive conditioning‐testing trials applied once every 7‐10 min over a 4‐16 hr perod. Each conditioning‐testing trial consisted of a 200‐600 impulse conditioning train followed by a series of testing impulses. The magnitudes and time constants of decay of augmentation and potentiation following each successive conditioning train were determined by measuring the e.p.p. amplitudes resulting from the testing impulses. 3. The magnitude of augmentation immediately following the conditioning trains increased an average of 3‐4 times (range 1‐20) with sucessive trials. 4. As the magnitude of augmentation increased with successive trials the decay of augmentation deviated from a simple exponential, decaying faster immediately after the conditioning train. This faster decay led to a 20% decrease with successive trials in estimates of the time constant obtained from the first 10 or 20 sec of the decay of augmentation. The deviation of the decay of augmentation from a simple exponential could be accounted for if augmentation is related to the 4th power of some substance which decays with a simple exponential time course. Some alternative explantations for the non‐exponential decay of augmentation are also discussed. 5. The magnitude of potentiation increased or decreased about 25% with successive trials. 6. The time constant characterizing the decay of potentiation inceased an average of 1‐5 times (range 0‐8‐5 times) with successive trials. 7. The increase in the magnitude of augmentation with successive trials was accompanied by a similar increase in the magnitude of the e.p.p. amplitudes during the conditioning trains, suggesting that augmentation develops during the conditioning train. In some preparatons augmentation appeared to be the major factor acting to increase e.p.p. amplitudes during the conditioning train, having a greater effect than facilitation or potentiation. 8. If a sufficiently large number of successive trials were applied, a depression of e.p.p. amplitudes developed during the conditioning trains and estimates of the magnitude of potentiation following the depressed conditioning trains were reduced...

1. End‐plate potential (e.p.p.s) were recorded with a surface electrode from frog neuromuscular junctions blocked with high Mg and low Ca to study post‐tetanic potentiation (potentiation). 2. The magnitude of potentiation was not directly related to the number of conditioning impulses, but was a function of the frequency and duration of the conditioning stimulation. 3. Potentiation was always greater following an equal number of impulses delivered at a higher frequency of stimulation. 4. Plots of the magnitude of potentiation against the number of conditioning impulses would sometimes show an upward inflexion depending on the parameters of stimulation. 5. These experimental observations were described by a model based on the assumption (1) that potentiation is linearly related to a residual substance, R(t), which accumulates in the nerve terminal during repetitive stimulation, and (2) that each nerve impulse adds an identical increment, r, of this residual substance. The data were not described by assuming a 4th power relationship between potentiation and R(t). 6. The upward inflexion in potentiation (see paragraph 4) is described by the model as resulting from an increase in the time constant for the decay of potentiation as the magnitude of potentiation increases. 7. The increment of residual substance r added by each impulse was independent of the amount of transmitter released during the conditioning train. This increment typically increased transmitter release by amount 1% of the control level in the absence of potentiation. 8. Suggestions are given to explain why potentiation of transmitter release, which is thought to arise from an accumulation of Ca‐2+ in the nerve terminal, can be described assuming a linear relationship between potentiation and R(t), the proposed substance responsible for potentiation, under experimental conditions in which a 3rd to 4th power relationship would be expected to exist between external Ca concentration and evoked transmitter release.

1. End‐plate potentials (e.p.p.s) were recorded with a surface electrode from frog neuromuscular junctions blocked with high Mg and low Ca to study post‐tetanic potentiation(potentiation). 2. Potentiation is found to decay exponentially over most of its time course. 3. The time constant tsu(H) characterizing this exponential decay is a function of the previous history (frequency and duration) of stimulation. For example, tsu(H) increased from about 20 sec following a few impulses to over 70 sec following more than 1000 impulses. 4. A new method is presented to obtain estimates of the rise of potentiation (uncontaminated by facilitation or an intermediate facilitatory process) during repetitive stimulation. It is found that potentiation is present following short trains of impulses and continually increases in magnitude with the duration of the conditioning stimulation. Potentiation was at a maximum immediately following the conditioning trains. 5. The relationship between P(T), the magnitude of potentiation immediately following repetitive stimulation, and tsu(H), the time constant for the decay of this potentiation, is given by tsu(H)=Ae‐P(T)/B, where A=19‐8 plus or minus 5‐1 sec (mean plus or minus S.D. of an observation) and B increases from 2.2plus or minus 2‐1 to 5‐7 plus or minus 2‐7 as the stimulation rate increases from 5 to 30/sec. 6. The value of A in the above equation can be considered to represent the minimal time constant for the decay of potentiation‐‐that is, the time constant for decay after a simgle impulse. 7. Evidence is presented for a afacilitatory process with a time constant of decay of about 3 sec which is intermediate in duration between facilitation and potentiation. 8. It is suggested that repetitive stimulation has a dual effect on potentiation; each impulse (1) adds an increment of potentiation and (2) increases tsu(H), the time constant for the decay of potentiation.

We induced the oversynthesis of elongation factors Tu and G by using multicopy plasmids carrying the structural genes for these proteins under the control of the lac operator-promoter. We found no evidence that accumulation of excess elongation factor Tu or G affects the expression of genes for ribosomal proteins or elongation factors.

1. Composite group Ia excitatory postsynaptic potentials (EPSPs) produced by heteronymous nerve stimulation were recorded from triceps surae motoneurons of barbiturate-anesthetized cats. Motoneuron rheobase, input resistance, and axonal conduction velocity were measured, and motor units were classified on the basis of the mechanical responses of their muscle units. 2. The amplitude of EPSPs recorded from 33 medial gastrocnemius (MG) motoneurons ranged from 0.6 to 4.3 mV. The mean EPSP amplitude differed among the major MG motor-unit types, increasing in the order fast twitch, fast fatiguing (FF); fast twitch, fatigue resistant (FR); slow twitch, fatigue resistant (S) (FF less than FR less than S). The amplitude of EPSPs recorded from 15 soleus motoneurons ranged from 0.3 to 3.4 mV, with a mean of 1.4 mV. 3. Presynaptic inhibition of EPSPs was produced by trains of conditioning volleys in the posterior biceps-semitendinosus (PBST) nerve. In 33 MG cells PBST conditioning stimulation reduced the amplitude of EPSPs by 11-50%, with a mean inhibition of 27%. The amplitude of EPSPs in 15 soleus motoneurons was decreased by 5-84%, with a mean inhibition of 37%. 4. When the magnitude of presynaptic inhibition was expressed as percent inhibition, there was no relation between presynaptic inhibition and either motor-unit type or the amplitude of the EPSP. However, when presynaptic inhibition was expressed as the absolute amount of inhibition in millivolts, the magnitude of inhibition was highly correlated with EPSP amplitude both across the entire triceps surae population (MG, lateral gastrocnemius, soleus) as well as within each muscle population. This correlation was also significant within the MG FF and FR motor-unit populations. 5. We conclude that EPSP amplitude and not motor-unit type is the major determinant of the magnitude of presynaptic inhibition. However, because of the effect of motor-unit type on EPSP amplitude, the net effect is that presynaptic inhibition increases in the order FF less than FR less than S.

Experiments were performed to test the possibility that motor unit-muscle spindle pairs that are coupled especially strongly mechanically will also be coupled especially strongly synaptically ("weighted ensemble input": Ref. 4). Synaptic and mechanical coupling between one or two individual muscle spindle afferents and individual motor units of the medial gastrocnemius (MG) muscle were measured in barbiturate-anesthetized cats. Synaptic coupling was assessed by measuring the amplitude of single-fiber monosynaptic excitatory postsynaptic potentials (EPSPs) generated in motoneurons by individual spindle afferents. Mechanical coupling was assessed by measuring the alteration in discharge rate of these spindle afferents caused by tetanic activation of the same motor units. Afferents were classified as primary or secondary on the basis of conduction velocity and response to muscle stretch and contraction. Motor units were classified as slow twitch (S); fast twitch, fatigue resistant (FR); fast twitch, intermediate fatigue resistance (FI); and fast twitch, fatigue sensitive (FF) on the basis of twitch contraction time and resistance to fatigue. In 85% of 138 motor unit-primary afferent interactions tested, tetanic activation of the single motor unit unloaded (i.e., decreased the discharge rate of) the primary afferent. A very weak though significant correlation was found between tetanic contraction strength and primary afferent unloading. In 66% of 155 motor unit-secondary afferent interactions tested, tetanic activation of the single motor unit unloaded the secondary afferent. Again, afferent unloading was but weakly related to tetanic contraction strength. Single-fiber EPSPs generated by primary or secondary muscle spindle afferents were recorded in type-identified motor units. EPSPs generated by primary afferents were significantly larger in oxidative (S + FR) than in glycolytic (FF) motor units. No such differences were seen for EPSPs generated by secondary afferents. The magnitude of the EPSP generated in a motoneuron by a spindle afferent was compared to the magnitude of the unloading of that afferent by tetanic activation of the corresponding motor unit. Overall, no relationship was found between these measures.(ABSTRACT TRUNCATED AT 400 WORDS)

The S10 ribosomal protein operon is regulated autogenously by the product of one of the genes of the operon, the gene encoding ribosomal protein L4. We have used site-directed mutagenesis to isolate leader mutations affecting L4 control. The phenotypes of these mutants demonstrate that L4 regulates both transcription and translation of the S10 operon. Several mutations abolish both levels of L4 control; others eliminate either transcriptional or translational control with little or no effect on the other mode of regulation. We conclude that L4-mediated transcriptional and translational control share some sequence requirements, but the two regulatory processes recognize somewhat different features of the S10 leader. Primary as well as secondary structures within the S10 leader appear to be involved.

We have cloned a DNA fragment harboring the genes for ribosomal proteins L2, L4, and L23 on a plasmid vector that contains a lac operator and promoter. The cloned ribosomal protein genes are now under the control of lacOP. Addition of a lac inducer to these cells results in a specific 5- to 10-fold increase in the synthesis of the proteins corresponding to the cloned genes. Within 10 min of this induction, the synthesis of ribosomal proteins S3, S19, L3, L16, L22, and L29 stops almost completely. The genes for all these proteins reside in the same chromosomal operon as L2, L4, and L23. We have seen no dramatic effect on the synthesis of any other ribosomal proteins. Thus, the induction of L2, L4, and L23 results in a specific and rapid decrease in the expression of all (or almost all) genes in their own transcription unit.

Mutations in the unc-52 gene on linkage group II retard the construction of body-wall muscle sarcomeres during larval development in the nematode Caenorhabditis elegans. Unc-52 mutants show decreased accumulation of myosin heavy chains relative to other polypeptides during larval development, correlating with the structural retardation. Pulse radiolabeling experiments show that decreased synthesis of specific body-wall myosin heavy chains that are encoded by the unc-54 gene on linkage group I is responsible for the defective myosin accumulation. In the wild type, a constant ratio of the synthesis of the unc-54-coded myosin B to myosin A, about 2:1, is maintained during the larval stages in which the synthesis of both myosins increases exponentially and rapid sarcomere growth and addition ensues. During the first 26 hr of larval development, before any structural or behavioral effects of unc-52 mutations are apparent, the synthesis of myosin heavy chains is also normal. By 38 hr, decreased synthesis of myosin B is detected in the unc-52 mutant SU200, when sarcomere growth slows considerably. The effects of mutation in the unc-52 locus are trans acting upon the synthesis of unc-54-coded myosin in a specific set of muscle cells during a defined period of larval development.

Endplate potentials were recorded from frog sartorius neuromuscular junctions under conditions of greatly reduced quantal contents to develop a quantitative description of stimulation-induced changes in transmitter release. Four general models relating potentiation, augmentation, and the first and second components of facilitation to transmitter release were developed. These models were then tested by incorporating equations for the kinetic properties of the four components of increased transmitter release and examining the ability of the resulting sets of equations to predict stimulation-induced changes in transmitter release. Three of the models were essentially consistent with the observation that augmentation had a multiplicative type relationship to facilitation. These models could also predict the effect of frequency and duration of stimulation on endplate potential (EPP) amplitude during and after prolonged (40 s) trains including the response to step changes in stimulation rate. These models extend by about two orders of magnitude the duration of stimulation-induced changes in transmitter release that can be accounted for, and show that the combined kinetic properties of potentiation, augmentation, and the two components of facilitation are generally sufficient to account for these changes.

Endplate potentials were recorded from frog and toad sartorius neuromuscular junctions under conditions of greatly reduced quantal contents. The magnitudes of augmentation increased with the duration and frequency of stimulation, often increasing at an accelerating rate during 10-20-s conditioning trains. The magnitudes of the first and second components of facilitation also increased, but reached apparent steady state values within the first few seconds of stimulation. These observations could be accounted for by assuming (a) that augmentation and the first and second components of facilitation arise from underlying factors in the nerve terminal that act to increase transmitter release; (b) that each nerve impulse adds an increment to each of the underlying factors; (c) that the magnitude of the increment typically increases during the train for augmentation but remains constant for the components of facilitation; and (d) that the underlying factors decay with first-order kinetics with time constants of approximately 7 s for augmentation and 60 and 500 ms for the first and second components of facilitation, respectively. The increments of facilitation added by each impulse were about twice as large in the toad as in the frog. Facilitation was described better by assuming a power relationship between the underlying factor and the observed facilitation than by assuming a linear relationship. Augmentation was described by assuming either a linear or power relationship.

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation-induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation-induced changes in MEPP frequency and EPP amplitude have similar time-courses.

Endplate potentials (EPP) were recorded from the frog sartorius neuromuscular junction under conditions of low quantal content to study the effect of Ba2+, Sr2+, and Ca2+ on the changes in evoked transmitter release that occur during and after repetitive stimulation. The addition of 0.1-1 mM Ba2+ or Sr2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with 0.8-1.4 mM Sr2+, led to a greater increase in EPP amplitudes during and immediately after repetitive stimulation. These changes in release were analyzed in terms of the four apparent components of increased transmitter release that have previously been distinguished on the basis of their kinetic properties. The Ba2+-induced increase in EPP amplitudes was associated with an increase in the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation or the first and second components of facilitation. The Sr2+-induced increase in EPP amplitudes was associated with an increase in the magnitude and the time constant of decay of the second component of facilitation. Sr2+ had little effect on potentiation, augmentation, or the first component of facilitation. The selective effects of Ba2+ on augmentation and of Sr2+ on the second component of facilitation were reversible and could be obtained in the presence of the other ion. The addition of 0.1-0.3 mM Ca2+ to the bathing solution had little effect on potentiation, augmentation, or the two components of facilitation. These results provide pharmacological support for the proposal that there are four different components of increased transmitter release associated with repetitive stimulation and suggest that the underlying factors in the nerve terminal that give rise to these components can act somewhat independently of one another.

The effect of repetitive stimulation on synaptic transmission was studied in the isolated superior cervical ganglion of the rabbit under conditions of reduced quantal content. Excitatory postsynaptic potentials (EPSP) were recorded with the sucrose gap technique to obtain estimates of transmitter release. Four components of increased transmitter release, with time constants of decay similar to those observed at the frog neuromuscular junction at 20 degrees C, were found in the ganglion at 34 degrees C: a first component of facilitation, which decayed with a time constant of 59 +/- 14 ms (mean +/- SD); a second component of facilitation, which decayed with a time constant of 388 +/- 97 ms; augmentation, which decayed with a time constant of 7.2 +/- 1 s; and potentiation, which decayed with a time constant of 88 +/- 25 s. The addition of 0.1-0.2 mM Ba2+ to the Locke solution increased the magnitude but not the time constant of decay of augmentation. Ba2+ had little effect on potentiation. The addition of 0.2-0.8 mM Sr2+ to the Locke solution appeared to increase the magnitude of the second component of facilitation. Sr2+ had little effect on augmentation or potentiation. These selective effects of Ba2+ and Sr2+ on the components of increased transmitter release in the rabbit ganglion are similar to the effects of these ions at the frog neuromuscular junction. Although the effects of Ba2+ and Sr2+ are similar in the two preparations, the magnitudes of augmentation and the second component of facilitation after a single impulse were about 6-10 times greater in the rabbit ganglion than at the frog neuromuscular junction. These results suggest that the underlying mechanisms in the nerve terminal that give rise to the components of increased transmitter release in the rabbit ganglion and frog neuromuscular junction are similar but not identical.

We have examined the translational coupling between the first two genes in the S10 ribosomal protein operon. We isolated mutations blocking the translation of the first gene of the operon, coding for S10, and monitored their effects on translation of the downstream gene, coding for L3. All of the mutations inhibiting S10 synthesis also affected the synthesis of L3. However, these experiments were complicated by decreased mRNA synthesis resulting from transcription polarity, which we could only partially eliminate by using a rho-100 strain. To completely eliminate the problem of transcription polarity and obtain a more accurate measurement of the coupling, we replaced the natural S10 promoter with a promoter used by the bacteriophage T7 RNA polymerase. As expected, the T7 RNA polymerase was not subject to transcription polarity. Using this system, we were able to show that a complete abolishment of S10 translation resulted in an 80% inhibition of L3 synthesis. Other experiments show that the synthesis of L3 goes up as a function of increasing S10 synthesis, but the translational coupling does not assure strictly proportional output from the two genes.

In the 5 to 10 min immediately following a shift from 30 to 42 degrees C, the differential synthesis rates of ribosomal proteins encoded by the 11-gene S10 operon are transiently decreased. This effect results largely from a two- to threefold decrease in the differential rate of transcription of the operon. The inhibition of mRNA synthesis is apparently due to two types of control: (i) initiation of transcription at the S10 promoter is inhibited and (ii) readthrough at the attenuator in the S10 leader is decreased. Both of these effects on transcription are independent of the heat shock regulatory gene, htpR. Furthermore, the inhibition of transcription is observed in both relA+ and relA cells, suggesting that the temperature-induced repression does not require the relA-dependent accumulation of guanosine tetraphosphate (ppGpp). However, recovery from the heat shock was slower in relA+ strains than in relA strains. None of the other ribosomal protein operons that we analyzed showed such a strong decrease in transcription after the heat shock.

The tufA gene, one of two genes in Escherichia coli encoding elongation factor Tu (EF-Tu), was cloned into a ColE1-derived plasmid downstream of the lac promoter-operator. In cells carrying this plasmid, the synthesis of EF-Tu was increased four- to fivefold upon the addition of isopropyl-beta-D-thiogalactopyranoside (an inducer of the lac promoter). This condition led to the synthesis of a novel protein, called pTu, which comigrated with EF-Tu on a sodium dodecyl sulfate-polyacrylamide gel but could be separated on an isoelectric focusing gel, since pTu is slightly more basic than EF-Tu. The synthesis of pTu could also be induced by the synthesis of a hybrid protein containing just the amino-terminal half of the EF-Tu protein. Genetic data suggest that pTu is the product of the tufA and tufB genes. The pTu protein was shown to be related to EF-Tu by gel electrophoresis of tryptic peptides. Pulse-chase experiments suggest that pTu is a precursor of EF-Tu. Interestingly, in a classic membrane fractionation procedure, EF-Tu was found in the cytosolic fraction, whereas pTu was partitioned with the outer membrane.

We have constructed a high-copy-number plasmid carrying an allele of the supD gene (supD43,74). The plasmid conferred temperature-sensitive suppression of amber mutations. Strains carrying the plasmid exhibited 50 to 60% suppression at 30 degrees C but little or no suppression at 42 degrees C. After a temperature shift from 30 to 42 degrees C the efficiency of suppression decreased gradually over a 60- to 90-min period before reaching the 42 degrees C steady-state level of suppression.

Some of the spontaneous streptomycin-resistant mutants of Escherichia coli strain C600 exhibit pleiotropic effects in addition to the antibiotic resistance. These effects include decreased growth rates, reduced levels of certain enzymes, and poor support of bacteriophage growth. One of these mutants, strain SM3, was studied further. We have examined the question of whether the reduced growth rate of the mutant SM3 is related to the reduction in relative amounts of ribosomes or to the reduction in the efficiency of ribosomes in protein synthesis. Measurements of alpha, the differential synthesis rate of ribosomal protein, revealed that the protein synthesis effeciency of ribosomes from the mutant strain SM3 was reduced about twofold relative to that of the parent strain C600. Measurements of the induction lag for beta-galactosidase and of the synthesis time of several different molecular-weight classes of proteins indicated that the mutation resulted in a marked reduction in the peptide chain growth rate. This reduction in the chain growth rate probably accounted for most of the observed reduction in the growth rate of the mutant strain. These experimental results show that the strA gene product, the S12 protein of the 30S subunit, is involved in some aspect of protein chain elongation. Presumably this involvement occurs during the messenger ribonucleic acid-directed binding of transfer ribonucleic acid to the ribosome.