Small noncoding RNAs (ncRNAs) are non-translated transcripts with lengths below 300 nucleotide residues. Regulation of cellular processes under the influence of these ncRNAs is the most various in eukaryotic cells, but numerous ncRNAs are also found in bacteria. One of the best-known small prokaryotic ncRNAs is 6S RNA - it has been detected in all branches of bacteria. Due to their conserved secondary structure including a large central "loop" flanked by long double-helical arms, 6S RNAs can bind holoenzymes of RNA polymerase (RNAP) and inhibit their activity. This inhibits transcription of many genes. According to data of comparative transcriptome analysis, the 6S RNA-dependent regulation of transcription affects the expression level of hundreds of genes involved in various cellular processes. 6S RNA has the unique feature of serving as a transcription template for the synthesis of short product RNAs (pRNAs) complementary to the central part of the molecule. The length and abundance of pRNAs vary depending on the physiological status of the cell. The synthesis of pRNAs is of great importance because it releases RNAP and provides reversibility of the inhibition. A similar mechanism has been described for the noncoding mouse B2 RNA that inhibits the activity of RNAP II. This finding can be taken as evidence for the common evolutionary origin of the ncRNA-dependent regulation of RNAP and its immense significance for cells. This review summarizes the state of knowledge about the main features and functions of 6S RNAs from various bacterial species with a special focus on the peculiarities of pRNA synthesis. The majority of functional insights on 6S RNAs have been gained for E. coli 6S RNA as the best-studied model system.
