Abstract
Background:
RNase R is unstable due to its acetylation and resulting binding of tmRNA-SmpB but is stabilized under stress conditions.
Results:
tmRNA-SmpB promotes RNase R proteolysis by stimulating binding of Lon and HslUV.
Conclusion:
Acetylation initiates a series of events ultimately leading to RNase R proteolysis.
Significance:
This work elucidates a novel regulatory mechanism previously unknown in bacteria.
RNase R, an important exoribonuclease involved in degradation of structured RNA, is subject to a novel mechanism of regulation. The enzyme is extremely unstable in rapidly growing cells but becomes stabilized under conditions of stress, such as stationary phase or cold shock. RNase R instability results from acetylation which promotes binding of tmRNA-SmpB, two
trans
-translation factors, to its C-terminal region. Here, we examine how binding of tmRNA-SmpB leads to proteolysis of RNase R. We show that RNase R degradation is due to two proteases, HslUV and Lon. In their absence, RNase R is stable. We also show, using an
in vitro
system that accurately replicates the
in vivo
process, that tmRNA-SmpB is not essential, but it stimulates binding of the protease to the N-terminal region of RNase R and that it does so by a direct interaction between the protease and SmpB which stabilizes protease binding. Thus, a sequence of events, initiated by acetylation of a single Lys residue, results in proteolysis of RNase R in exponential phase cells. RNase R in stationary phase or in cold-shocked cells is not acetylated, and thereby remains stable. Such a regulatory mechanism, dependent on protein acetylation, has not been observed previously in bacterial cells.
