Fine structure analysis of Salmonella typhimurium glutamate synthase genes

Abstract

Glutamate synthase activity is required for the growth of SalmoneUa typhimurium on media containing a growth-rate-limiting nitrogen source. Mutations that alter glutamate synthase activity had been identified in the glitB gene, but it was not known which of the two nonidentical subunits of the enzyme was altered. To examine the gene-protein relationship of the gil region, two nonsense mutations were identified and used to demonstrate that gitB encodes the large subunit of the enzyme. Six strains with independent Mu cts dl(lac bla) insertions were isolated, from which a collection of deletion mutations was obtained. The deletions were transduced with the nonsense mutations and 38 other gil point mutations to construct a fine-structure genetic map. Chromosome mobilization studies, mediated by Hfr derivatives of Mu cts dl lysogens, showed that gItB is transcribed in a clockwise direction, as shown in the S. typhimurium linkage map. Studies of the polar effects of three Mu cts dl insertions indicated that the gene for the small subunit maps clockwise to gltB and that the two genes are cotranscribed to form a gi operon.