STUDY IN EVOLUTION - HISTIDINE UTILIZATION GENES OF ENTERIC BACTERIA

The histidine utilization (hut) system, MIGCPUH, comprises two operons, whose promoters are M and P. The DNA segments of phage λ vectors coding for the hut genes of Salmonella typhimurium and Klebsiella aerogenes were compared by using electron microscopy to study the nature of the hut DNA homology. The hut S. typhimurium/hut K. aerogenes DNA heteroduplexes were spread and mounted for electron microscopy under more and more denaturing conditions. The homology between hut S. typhimurium and hut K. aerogenes is extensive but, as denaturing conditions make base-pairing more difficult, the regions of lower homology are melted, while the regions of higher homology remain base-paired. The denaturation map could be correlated with the genetic map by the use of λhut S. typhimurium phages carrying different portions of the hut genome. The sequences with the highest homology are in the structural genes for the four enzymes, while the regulatory regions, the promoters and the hut repressor gene, are much less homologous. © 1979.