Isoleucine-valine Requiring Mutants of Salmonella Typhimurium.

initial reports on the mutant strains of Salmonella typhimurium that T: eui r both isoleucine and valine for growth (ilva strains) were published concomitantly by GLANVILLE and DEMEREC ( 1960) and WAGNER and BERGQUIST (1960). These studies showed that although the available ilva strains could be divided into four complementation groups ( A , B , C and D ) on the basis of abortive transduction results, only three loci, as determined by enzymatic analysis, were involved. These results are illustrated in Figure 1. Both the iZvaA strains and the single ilvaD strain are deficient in reductoisomerase. Various attempts to establish definite biochemical differences between these two groups have not been successful ( ARMSTRONG and WAGNER 1962) , and it is currently thought that the iZvaA and ilvaD designations may represent complementing subunits within the locus that controls the production of the reductoisomerase. A detailed discusson of complementation within a locus is found in the report by AMES and HARTMAN (1962) on the genetics of histidine biosynthesis in S. typhimurium. The transduction studies further showed that the tranducing fragment bearing the ilva loci also contains the locus (&A) that controls the production of a-ketobutyrate, a precursor of isoleucine (Figure 1 ) . Since the appearance of these reports a number of new iZva strains have been isolated by DR. M. DEMEREC and his associates, and the present publication is