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We have designed a new medium for the differentiation of mutants of Salmonella typhimurium defective in the ability to reduce nitrate with formate, and have characterized 24 formate dehydrogenase (FDH) mutants isolated on this medium. The mutants were assayed for the ability to use formate to reduce benzyl viologen and phenazine methosulfate, and were mapped by means of conjugation and P22-mediated transduction. Mutants lacking the ability to reduce either dye were found to map at three distinct sites: at a site co-transducible with xyl (presumably fdhA), at a site or sites between 13U and 33U, but not co-transducible with aroA, bio, purB, pyrC, or pyrD (near, but not identical with fdhB), and at asite 10-20% co-transducible with pyrE, for which we suggest the designation fdhC. Six mutant isolates reduced benzyl viologen, but not phenazine methosulfate. They retained the ability to produce nitrite during growth with nitrate. They mapped between 83U and 89U, but no co-transduction was found with metE, glnA, metB, or argH. The combined biochemical and genetic data suggest the existence of a gene in this area which is essential for the reduction of nitrate with formate, but not for formate hydrogenlyase activity or for nitrate reductase activity.

Cysteine auxotrophs of Salmonella typhimurium mutated in cysB, cysI or cysJ grew with sulphate as a sulphur source when incubated under a hydrogen/carbon dioxide atmosphere. Yields obtained under these conditions were equivalent to those characteristic of wild-type S. typhimurium. The same mutants failed to grow with sulphate as a sulphur source when incubated aerobically. Auxotrophs mutated in cysA, cysC, cysD, cysE, cysG and cysH required cysteine for growth under both incubation conditions. The results suggest that mutations in cysB (regulation of the several cys operons) and also cysI and cysJ (sulphite reductase activity) can be circumvented during anaerobic growth under hydrogen.

The hisT gene, one of six genes in which mutation causes derepression of the histidine operon in Salmonella typhimurium, is shown to code for a protein that is not essential for the growth of the bacteria. This is indicated by the characterization of particular classes of mutations in the hisT gene: amber mutations, frame-shift mutations, and temperature-sensitive mutations that affect repression but not growth. In addition, the class of semilethal mutations was selected for but not found.

In the course of selection of bacteria with derepressed levels of histidine biosynthetic enzymes, it was found that when mutagen-treated cells were spread on a selective medium without allowing intervening growth to occur, the frequency of operator mutants obtained was dramatically increased. This may be useful as a general enrichment for operator or other dominant mutations.