MYCOPLASMA BOVIS RIBOSOMAL RNA GENE CLUSTERS ARE HOT SPOTS FOR INSERTION SEQUENCES

Insertion sequences (ISs) are mobile genetic elements that can have a significant impact on genomic organization, coding capacity and gene expression of bacteria. IS elements are associated with genomic rearrangements, gene disruptions, virulence acquisition and hence are important elements in bacterial genome evolution. It has been previously shown that ISs represent about 7% of the complete genome of the bovine pathogen Mycoplasma bovis type strain PG45. No ISs were identified within the two tandem ribosomal RNA operons (rrn) of this strain. However, molecular characterization of the rrn locus in 70 M. bovis field isolates revealed variation in the copy number of rrn as well as the presence of ISs, related to the ISMbov1 and ISMbov4 isomers, in 35/70 isolates. The insertion of ISs occurred into intergenic regions (IGR)-1 or 3, comprising the putative promoter regions of rrn3 and rrn4, respectively and into IGR-5, located downstream to the 23S rRNA (rrl4) gene. Overall, seven different configurations (A-G) of the rrn locus with respect to ISs were obtained, including those found in three annotated genomes, suggesting that rrn locus serves as a hot spot for IS resulting in M. bovis intraspecies variability. The promoter region and the transcriptional start site (TSS) for rrn were further examined in M. bovis strain 88127, which contains only one rrn. TTS was mapped as a guanine nucleotide within the IGR-1, 60 bp upstream of the rrs gene. In addition, several shorter transcripts were identified indicating the presence of processed and maturated products. Since ISs may influence gene expression though their regulatory elements, the impact of the IS insertions within rrn-promoter region on expression of M. bovis ribosomal genes is currently under investigation.