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We have studied the ribonucleosidediphosphate reductase of a deoxyuridine-requiring mutant LD195 of Escherichia coli K. This enzyme was purified 50 fold from the mutant and a control strain. In all steps of the purification the mutant strain yielded normal activity of the Bi subunit of the enzyme but approximately 10 % of the wild-type activity of the B2 subunit The activity is similarly reduced for all three substrates tested CDP, UDP, and GDP. The mutation appears to affect the physical structure of the B2 subunit, since addition of high concentrations of sodium acetate can restore enzymatic activity to the partially purified B2 subunit from the mutant strain. In contrast, the same concentrations strongly inhibit the wild- type B2 subunit. Although the mutant has a requirement of deoxyuridine at 42 °C but not at 30 °C (as shown in an accompanying paper), mutant protein B2 is not more thermo-labile than wild-type protein B2 This requirement therefore probably reflects an increased demand of deoxyribonucleotides at higher temperatures. Hydroxyurea inhibits mutant and wild-type ribonucleosidediphosphate reductase to the same extent in vitro. In spite of this, hydroxyurea inhibits growth of the mutant strain at a 20 fold lower concentration than that required for comparable inhibition of growth of the parent. This is explained by the greatly decreased protein B2 activity of the mutant. [ABSTRACT FROM AUTHOR]

Transhydrogenase catalyses the transfer of reducing equivalents between NAD(H) and NADP(H) coupled to the translocation of protons across a membrane. The NAD(H)-binding domain of transhydrogenase (domain I protein) from Rhodospirillum rubrum and from Escherchia coli were overexpressed and purified. Nucleoude binding to the domain I proteins was determined by equilibrium dialysis. NADH and its analogue, acetylpyridine adenine dinucleotide (reduced form), bound with relatively high affinity (Kd = 32 μM and 120 μM). respectively, for the R. rubrum protein). The binding affinity was similar at pH 8.0 and pH 9.0 in zwitterionic buffers and at pH 7.5 in sodium phosphate buffer, NAD+hound with lower affinity (Kd = 300 μM). NADPH bound only very weakly (Kd1> 1 mM). Using a centrifugation procedure, Yamaguchi and Hatefi [Yamaguchi, M & Hatefi, Y. (1993) J. Biol. Chem, 268, 1781-17877 ] found that mitochondrial transhydrogenase, and a proteolytically derrived domain I fragment from that exnzyme, bound one NADH per dimer. They suggested that this result implied half-of-the-sites reactivity for the interaction between the nucleotide ligand and the protein. However our studies on both the E. coli and the R. rubrum recombinant transhydrogenase domain I protein using equilibrum dialysis show that the binding stoichiometry for both NADH and the reduced form of acetylpyridine adenine dinucleotide (AcPdADH) is two nucleotides per dimer: no interaction between the monochondrial transhydrogenases are discussed. [ABSTRACT FROM AUTHOR]

The sequence of the tufA gene from the extreme thermophilic eubacterium Thermus aquaticus EP 00276 was determined. The GC content in third positions of codons is 89.5%, with an unusual predominance of guanosine (60.7%). The derived protein sequence differs from tufA- and tufB- encoded sequences for elongation factor Tu (EF-Tu) of Thermus thermophilus HB8, another member of the genus Thermus, in 10 of the 405 amino acid residues. Three exchanges are located in the additional loop of ten amino acids (182–191 ). The loop, probably involved in nucleotide binding, is absent in EF-Tu of the mesophile Escherichia coli. Since EF-Tu from E. coil is quite unstable, the protein is well-suited for analyzing molecular changes that lead to thermostabilization. Comparison of the EF-Tu domain I from E. coli and Thermus strains revealed clustered amino acid exchanges in the C-terminal part of the first helix and in adjacent residues of the second loop inferred to interact with the ribosome. Most other exchanges in the guanine nucleotide binding domain are located in loops or nearest vicinity of loops suggesting their importance for thermostability. The T. aquaticus EF-Tu was overproduced in E. coil using the tac expression system. Identity of the recombinant T. aquaticus EF-Tu was verified by Western blot analysis, N-terminal sequencing and GDP binding assays. [ABSTRACT FROM AUTHOR]

cAMP receptor protein from Escherichia coli has been purified on a large scale. Analogues of cAMP modified on the 6-NH2 group of the adenosine ring, the ribose 2'OH group or the cyclic phosphate are able to displace cAMP from its binding site with dissociation constants of similar magnitude to that of cAMP. More extensive modification produces weaker binding. Ultraviolet/visible difference spectroscopy and fluorescence spectroscopy show that the environment of the bound adenosine moiety is considerably less polar than that in aqueous solvent, while an anthraniloyl group substituted on the 2'OH position remains accessible to solvent. The 2-NH2 group of cGMP appears to be protonated in the bound form, while no change in the charge state of cAMP is apparent. [ABSTRACT FROM AUTHOR]

The replication of M13 single-stranded DNA by the 9S DNA polymerase α from calf thymus has been studied in vitro. Priming conditions, the nature of the replication products and conditions for optimal elongation have been investigated. Oligonucleotides comprising only four nucleotides can serve as primers. Both ribo and deoxy oligonucleotides can be elongated. Priming by the short oligonucleotides occurs at multiple sites on the M13 genome. If replication is primed at single sites with a specific pentadecamer or with RNA in the origin of replication, specific pausing sites are observed. These pausing sites can partly be correlated with secondary structures in the template DNA. Addition of Escherichia coli single- stranded DNA binding protein leads to a weakening of pausing sites and to the synthesis of longer products. The 9S enzyme is able to proceed through most of the pausing sites resulting in the synthesis of product molecules as long as 6600 nucleotides. The 9S DNA polymerase α contains a potent DNA primase activity which enables it to initiate replication on a single-stranded template in the presence of the four NTPs . However, priming is also possible in the presence of ATP alone. The priming sites are not randomly distributed over the M13 DNA. [ABSTRACT FROM AUTHOR]

Limited digestion of the Escherichia coli 50-S subunit permits the isolation of two large fragments of the 23-S RNA. One arises from the 5' end of the 23-S RNA and contains about 1200 nucleotides. The other arises from the 3' end of the 23-S RNA and contains about 2000 nucleotides. Each of the ten 50-S proteins known to bind specifically and independently to the 23-S RNA were tested for their ability to interact with these two RNA fragments. It was determined that the 5' terminal segment contains the specific binding sites for proteins L4, L20 and L24, whereas the 3' terminal segment contains the specific binding sites for proteins LI, L2, L3, L6, L13, L16 and L23. [ABSTRACT FROM AUTHOR]