VARIATION OF THE RIBOSOMAL OPERON 16S-23S GENE SPACER REGION IN REPRESENTATIVES OF SALMONELLA-ENTERICA SUBSPECIES

The 16S-23S spacer regions of two ribosomal operons (rrnA and rrnE) ha ve been sequenced in seven representatives of the Salmonella enterica subspecies. Isolated nucleotide substitutions were found at the same s ites as in Escherichia coli but the number of polymorphic sites was mu ch larger, as could be expected for a more heterogeneous species. Stil l, as in E. coil, most of the variation found was due to insertions an d/or deletions affecting blocks of nucleotides generally located at eq uivalent regions of the putative secondary structure for both species. Isolated polymorphic sites generated phylogenetic trees generally con sistent with the subspecies structure and the accepted relationships a mong the subspecies. However, the sequences of rrnE put subspecies I c loser to E. coli K-12 than to the other S. Enterica subspecies. The di stribution of polymorphisms affecting blocks of nucleotides was much m ore random, and the presence of equivalent sequences in distantly rela ted subspecies, and even in E. coli, could reflect relatively frequent horizontal transfer. The smallest 16S-23S spacers in other genera of the family Enterobacteriaceae were also sequenced. As expected, the le vel of variation was much larger. Still, the phylogenetic tree inferre d is consistent with those of 16S rRNA or housekeeping genes.