Role of 4.5S RNA in assembly of the bacterial signal recognition particle with its receptor

The mechanism by which a signal recognition particle (SRP) and its receptor mediate protein targeting to the endoplasmic reticulum or to the bacterial plasma membrane is evolutionarily conserved. in Escherichia coli, this rea ction is mediated by the Ffh/4.5S RNA ribonucleoprotein complex (Ffh/4.5S R NP; the SRP) and the FtsY protein (the SRP receptor). We have quantified th e effects of 4.5S RNA on Ffh-FtsY complex formation by monitoring changes i n tryptophan fluorescence. Surprisingly, 4.5S RNA facilitates both assembly and disassembly of the Ffh-FtsY complex to a similar extent. These results provide an example of an RNA molecule facilitating protein-protein interac tions in a catalytic fashion.