Characterization of the three ribosomal RNA operons rrnA, rrnB, and rrnC, from Brucella melitensis

The three Brucella melitensis ribosomal RNA operons rrnA, rrnB, and rrnC were characterized individually. Each locus consisted of the 16S rRNA gene (rrs), followed by an intergenic spacer containing the tRNA-Ile and tRNA-Ala genes, the 23S rRNA gene (rrl), an intergenic spacer devoid of tRNA genes, the 5S rRNA gene (rrf), and an f-Met tRNA gene. The DNA sequences were identical over a 6271bp region, diverging 594bp upstream of rrs and immediately downstream of the f-Met tRNA gene. The previously uncharacterized 23S rRNA genes each contained a 178bp insertion 130bp from the 5' end. The location of the insertion matched intervening sequences (IVSs) found in other Rhizobiaceae. However, the size and sequence of the Brucella IVS differed from all previously reported IVS sequences from bacteria. The IVS region was PCR-amplified from 20 Brucella isolates representing all known Brucella species and biovars. All isolates contained only the complete IVS fragment. We compared the IVS DNA sequences of rrlC from representative strains of each of the six known Brucella species. The data revealed that the sequences were identical and differed from the B. melitensis IVS sequences by a single base pair. In other bacterial species, the IVSs are associated with post-transcriptional processing of the 23S rRNA by RNase III. We found that the Brucella 23S rRNA was slightly smaller than the 23S rRNA of Escherichia coli, known to be devoid of IVS sequences.