Mechanism of methionine synthase overexpression in chaperonin-depleted Escherichia coli

The chaperonin GroE (GroEL and the co-chaperonin GroES) is the only chaperone system that is essential for the viability of Escherichia coli. GroE is absolutely required for the folding of at least 57 proteins in E. coli, referred to as class IV substrates, and assists in the folding of many more. Although GroE is mainly involved in protein folding, when it is depleted, the expression levels of about a hundred further proteins can be seen to increase, most prominently methionine synthase (MetE). Here we investigate the mechanism of metE overexpression in GroE-depleted cells. Gene fusion experiments in which the metE transcriptional region was fused to an assayable reporter showed that addition of a GroE-independent MetK homologue [MetK synthesizes S-adenosylmethionine (SAM), the metJ corepressor] to the system (E. coli MetK depends on GroE for folding) almost fully suppressed the increased expression. An analysis of deletion mutants in the metE promoter, and overexpression and disruption of the metR gene, showed that the absence of MetJ binding and increased levels of the activator MetR resulted in the overexpression of MetE. We conclude that the need of metE for metK, and the need of metK for GroE, can explain the overexpression of methionine synthase in GroE-depleted cells.