CHROMOSOMAL REARRANGEMENTS IN ENTERIC BACTERIA

Early genetic studies showed conservation of gene order in the enteric bacteria. Two recent methods using pulsed field gel electrophoresis ( PFGE) to determine the physical map of the genome are: (i) partial dig estion with the endonuclease I-CeuI, which digests the DNA of bacteria in the rrn operon for rRNA (ribosomal RNA), thus establishing the ''r rn genomic skeleton'' (the size in kbp of the intervals between rRNA o perons); (ii) analysis of XbaI and BlnI sites within Tn10 insertions i n the chromosome. The order of I-CeuI fragments, which is ABCDEFG in S . typhimurium LT2 and E. coli K-12, was found to be conserved in most Salmonella species, most of which grow in many hosts (host-generalists ). However, in S. typhi, S. paratyphi C, S. gallinarum, and S. pulloru m, species which are host-specialized, these fragments are rearranged, due to homologous recombination between the rrn operons, resulting in translocations and inversions. Inversions and translocations not invo lving the nn operons are seldom detected except for inversions over th e TER (termination of replication) region. Additive genetic changes (d ue to lateral transfer resulting in insertion of nonhomologous DNA) ha ve resulted in ''loops'' containing blocks of DNA which provide new ge nes to specific strains, thus driving rapid evolution of new traits.