In addition to ribosomal RNA genes, a relatively small number of genes are highly conserved between Escherichia coli and Salmonella typhimurium on the one hand, and either Serratia marcescens or Proteus morganii on the other hand. The conserved non-rRNA genes are not the same in S. mareescens and P. morganii. Different genetic segments have maintained nucleotide sequence similarity to parts of the E. coli and S. typhimurium genemes in the course of the evolution of S. marcescens and P. morganii.
Only a very small fraction of either Serratia or Proteus DNA is closely similar in nucleotide sequence to Escherichia DNA. Using stringent conditions of hybridization, only 3% of Serratia marcescens DNA and 1% of Proteus mirabilis DNA form hybrid duplexes with Escherichia coli DNA (Brenner et al. 1969;Brenner 1973). To learn more about the small fraction of highly conserved sequences in enterobacterial genomes, we have examined the hybrid patterns produced when E. coli chromosomal DNA, Salmonella typhimurium chromosomal DNA, E. coIi F' DNAs and E. coli ribosomal RNA were used as probes to hybridize to EcoRI restriction endonuclease digests of S. marcescens and P. morganii chromosomal DNA's. The four F' DNAs used in these experiments, F'128, F'133, F'148 and F'152 carry two to four map units each of E. coli chromosomal DNA from four widely separated regiorLs of the genome (Low 1972;Bachmann and Low 1980).