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A further series of mutant am alleles, encoding potentially active NADP-specific glutamate dehydrogenase (GDH) and capable of complementation in heterocaryons, have been characterized with respect to both GDH properties and DNA sequence changes. Several mutants previously studied, and some of their same-site or second-site revertants, have also been sequenced for the first time. We present a summary of what is known of the properties of all am mutants that have been defined at the sequence level.

SUMMARYCurrently favoured models postulate that gene conversion is due to the correction of mis-matches in heteroduplex DNA. If heteroduplex is formed reciprocally on both chromatids participating in recombination, the mis-matches due to a heterozygous site will be different on the two chromatids, and there will be four correction probabilities to be taken into account. It is shown that, given the frequencies of the five different kinds of aberrant ascus ratios, it is possible to calculate four alternative sets of values for the four correction probabilities and the total number of asci in which heteroduplex is formed. These four solutions reduce in effect to two when there are no other markers distinguishing the two chromatids. With the aid of flanking markers and the assumption that heteroduplex formation is chemically polarized, it is possible, in principle, to choose one best solution.The method has been applied to the five one-point crosses inSordaria fimicolafrom which most data are available. The data from four different mutants crossed to wild type are compatible with a restricted model in which the correction frequencies, from mutant to wild and from wild to mutant, are the same on both chromatids. In the case of the fifth mutant the data are not consistent with this restricted model, and indicate different correction frequencies in the two chromatids.

A total of eighty-seven revertants were induced by ultra-violet light in anam3strain. All of these revertants appear to be the result of mutation at sites in or close to theamlocus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and elevenammutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of theammutants, some witham1only, and some witham1or witham7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in anamstrain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.

SUMMARYMutants ofUstilago maydishave been isolated both, deficient and derepressed for nitrate reduction. Those deficient in enzyme fall into six groups, one of which is the structural gene. Enzyme which has proved to be more labile than that of wild-type has been isolated from a temperature-sensitive mutant at this locus. All the mutants in the structural gene have xanthine dehydrogenase activity and the situation closely parallels that ofAspergillus nidulans.The derepressed mutants fall into four complementation groups and all are partially derepressed in that they are further inducible by nitrate. Full derepression can be conferred by induction of a second mutation. In one analysed case the second reinforcing mutation proved to be pheno-typically similar to the first one when separated from it.

Maps have been made showing the order ofammutant sites using (a) frequencies ofam+recombinants from crosses betweenammutants and (b) the modes of distribution of theam+recombinants among the two parental and two recombinant classes with respect to the flanking markersinosandsp.It is possible to arrange the sites in an order such that, in almost all the crosses yielding useful numbers ofam+recombinants, thespallele originally in coupling with the distalammutant site occurs in the majority of the recombinants. No such consistent pattern was found with respect to theinosmarker. The order obtained by reference tospagreed with the best order deducible from recombination frequencies.The data are consistent with the hypothesis thatam+recombinants arise by a process of gene conversion, that there is a gradient of conversion frequencies from the right (inosor distal) end of the gene to the left (spor proximal) end, and that conversion tends to be associated (though less than 50% of the time) with crossing-over, especially on the distal side.There is no obvious relationship between the map position of a givenammutant and the properties of the protein product of the mutant gene.

Six newammutants ofNeurospora,am14–am19, with defective formation of NADP-linked glutamate dehydrogenase, were isolated following treatment of conidia with nitrous acid. This brings the number of genetically distinct and viableammutants to sixteen. Heterocaryon-compatible isolates of each of the new mutants were tested for complementation with each of the mutants previously known. A number of new complementary combinations were found which add to the complexity of the complementation map which, however, remains linear.The mutants which show complementation aream1,am2,am3,am7,am14andam19. It has proved possible to isolate an altered form of glutamate dehydrogenase from each of these exceptam14, which appears to produce no protein fractionating like the normal enzyme. Among the non-complementing mutants,am4produces an easily-isolated inactive form of glutamate dehydrogenase. These results agree with the immunochemical findings of Roberts & Pateman (1964).Of the mutationally altered forms of the enzyme, theam1,am2,am3andam7varieties are electrophoretically identical or closely similar to the normal protein, while theam4andam19varieties move faster towards the anode at pH 8·5,am19being the faster of the two. Theam19protein, like theam2andam3proteins previously reported on, can be activated to give some enzyme activity, but the activity obtained is much lower for this mutant than foram2oram3.All the complementary pairs of mutants, including those involvingam19, form active enzyme varieties which have very close to normal electrophoretic properties. In several cases the complementation enzyme is less stable to heat than the wild-type enzyme.