<P> Amongst the targets of sRNAs are a number of house - keeping genes . The 6S RNA binds to RNA polymerase and regulates transcription, tmRNA has functions in protein synthesis, including the recycling of stalled ribosomes, 4.5 S RNA regulates signal recognition particle (SRP), which is required for the secretion of proteins and RNase P is involved in maturing tRNAs . </P> <P> Many sRNAs are involved in stress response regulation . They are expressed under stress conditions such as cold shock, iron depletion, onset of the SOS response and sugar stress . The small RNA nitrogen stress - induced RNA 1 (NsiR1) is produced by Cyanobacteria under conditions of nitrogen deprivation . Cyanobacteria NisR8 and NsiR9 sRNAs could be related to the differentiation of nitrogen - fixing cells (heterocysts). </P> <P> The RpoS gene in E. coli encodes sigma 38, a sigma factor which regulates stress response and acts as a transcriptional regulator for many genes involved in cell adaptation . At least three sRNAs, DsrA, RprA and OxyS, regulate the translation of RpoS . DsrA and RprA both activate RpoS translation by base pairing to a region in the leader sequence of the RpoS mRNA and disrupting formation of a hairpin which frees up the ribosome loading site . OxyS inhibits RpoS translation . DsrA levels are increased in response to low temperatures and osmotic stress, and RprA levels are increased in response to osmotic stress and cell - surface stress, therefore increasing RpoS levels in response to these conditions . Levels of OxyS are increased in response to oxidative stress, therefore inhibiting RpoS under these conditions . </P> <P> The outer membrane of gram negative bacteria acts as a barrier to prevent the entry of toxins into the bacterial cell, and plays a role in the survival of bacterial cells in diverse environments . Outer membrane proteins (OMPs) include porins and adhesins . Numerous sRNAs regulate the expression of OMPs . The porins OmpC and OmpF are responsible for the transport of metabolites and toxins . The expression of OmpC and OmpF is regulated by the sRNAs MicC and MicF in response to stress conditions . The outer membrane protein OmpA anchors the outer membrane to the murein layer of the periplasmic space . Its expression is downregulated in the stationary phase of cell - growth . In E. coli the sRNA MicA depletes OmpA levels, in Vibrio cholerae the sRNA VrrA represses synthesis of OmpA in response to stress . </P>

Rna acts at both the genetic and the functional levels