Transcriptional and regulatory control of the three-rRNA (rrn) operons of the nodulating symbiont Sinorhizobium meliloti

Date of Completion

January 2007


Biology, Molecular|Biology, Microbiology




In order to study the transcriptional expression of ribosomal RNA in the nodulating symbiont Sinorhizobium meliloti, the three ribosomal rRNA promoter regions (prrn1, prrn2 and prrn3) were fused to the gfp( mut3) reporter gene. prrn::gfp( mut3) reporter fusions were inserted into the low copy plasmid vector, pHC41. The rrn::gfp reporters were inserted into S. meliloti wild-type Rm1021 bacterial cells. ^ Promoter fusions indicated that the three ribosomal rRNA promoter regions were differentially expressed and regulated when the cells were free-living and during symbiosis. Transcriptional expression of prrn1 was stronger than prrn2 and prrn3 and its expression level did not decrease during early-exponential growth. Whereas the expression levels for prrn2 and prrn3 decreased during early-exponential growth. ^ In S. meliloti, the RNA/protein ratios of steady-state cells grown at different growth rates were not significantly different. This indicates that the synthesis of rRNA was not growth-rate controlled, at least when the cells were grown between doubling times of 0.30 hr-1 and 0.16 hr-1. Transcriptional expression of prrn1:: gfp and prrn2::gfp fusions did not vary with the cells' growth rate. The only promoter fusion that showed growth-rate controlled was prrn3::gfp. ^ In S. meliloti when the region upstream of the UP element from prrn2 and prrn3 was removed, promoter activity increased about 2.5 to 3 fold. These results suggest that the region upstream of the AT-rich UP element negatively regulated the transcriptional expression of both prrn2 and prrn3. Also, this region is responsible for the differences in the transcriptional expression of the three rrn promoter regions. ^ In summary, the differential expression patterns of the three rrn promoter regions in S. meliloti, the early down regulation of prrn2::gfp and prrn 3::gfp during early exponential phase, and the involvement of inhibitory cis-acting sequences upstream of the AT-rich UP element suggests that the mechanisms that regulate rRNA synthesis in this organism may be different from the mechanisms known to regulate rRNA synthesis in E. coli. and other well studied bacteria. ^