Comparison of the small 16S to 23S intergenic spacer region (ISR) of the rRNA operons of some Escherichia coli strains of the ECOR collection and E. coli K-12.

Several 16S to 23S spacers of 354 bp have been sequenced from six Escherichia coli strains belonging to the ECOR collection. Four phylogenetically informative variable sites were identified. The results of their comparison confirm the existence of two major phylogenetic branches in this species, as previously reported. Remarkable intercistronic heterogeneity was found in strain ECOR35 and its closest relatives, in which at least one of the operons has suffered a major mutagenic event or has an independent phylogenetic origin.