In molecular biology the 6S RNA is a noncoding RNA that was one of the first to be identified and sequenced. What it does in the bacterial cell was unknown until recently. It consists of 184 nucleotides that fold into an extended hairpin structure with a large single-stranded internal bulge.
6S RNA specifically associates with RNA polymeraseholoenzyme containing the sigma70 specificity factor. This interaction represses expression from sigma70-dependent promoters during stationary phase. 6S RNA homologs have recently been identified in most bacterial genomes. Many Gram-positive species have two copies of 6S RNA. In Bacillus subtilis, both copies appear to interact with RNA polymerase holoenzyme containing the housekeeping sigma factor and be expressed during different stages of growth. In many proteobacteria, 6S RNA may be processed from a transcript encoding homologs of the E. coli YgfA protein which is a putative methenyltetrahydrofolate synthetase. Diverged 6S RNAs have been identified in additional bacterial lineages. The purD RNA motif has been experimentally shown to overlap with 6S RNA.
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