The YidC/Oxa1/Alb3 family proteins are involved in membrane protein biogenesis in bacteria, mitochondria, and chloroplasts. Recent studies show that YidC uses a channel-independent mechanism to insert a class of membrane proteins into the membrane.Bacillus subtilishas two YidC homologs, SpoIIIJ (YidC1) and YidC2 (YqjG); the former is expressed constitutively, while the latter is induced when the SpoIIIJ activity is compromised. MifM is a substrate of SpoIIIJ, and its failure in membrane insertion is accompanied by stable ribosome stalling on themifM-yidC2mRNA, which ultimately facilitatesyidC2translation. While mutational inactivation of SpoIIIJ has been known to induceyidC2expression, here, we show that the level of this induction is lower than that observed when the membrane insertion signal of MifM is defective. Moreover, this partial induction of YidC2 translation is lowered further when YidC2 is overexpressed intrans. These results suggest that YidC2 is able to insert MifM into the membrane and to release its translation arrest. Thus, under SpoIIIJ-deficient conditions, YidC2 expression is subject to MifM-mediated autogenous feedback repression. Our results show that YidC2 uses a mechanism that is virtually identical to that used by SpoIIIJ; Arg75 of YidC2 in its intramembrane yet hydrophilic cavity is functionally indispensable and requires negatively charged residues of MifM as an insertion substrate. From these results, we conclude that MifM monitors the total activities of the SpoIIIJ and the YidC2 pathways to control the synthesis of YidC2 and to maintain the cellular capability of the YidC mode of membrane protein biogenesis.
A ribosome–nascent chain sensor of membrane protein biogenesis in Bacillus subtilis
