OSMO-SENSING BY N-TERMINAL AND C-TERMINAL EXTENSIONS OF THE GLYCINE BETAINE UPTAKE SYSTEM BETP OF CORYNEBACTERIUM-GLUTAMICUM

The major uptake carrier for the compatible solute glycine betaine in Corynebacterium glutamicum is the secondary transport system BetP. It is effectively regulated by the external osmolality both on the level of expression and of activity. BetP carries highly charged domains bot h at the N and at the C terminus. We investigated the role of these ex tensions in the regulatory response to hyperosmotic stress. Mutants of the betP gene coding for proteins with truncated N- and C-terminal ex tensions were expressed in the C. glutamicum betP deletion strain DHP1 and were functionally characterized with respect to regulation of act ivity. The optimum of activation at 1.3 osmol/kg in wild type was shif ted in the recombinant strains to about 2.6 osmol/kg in mutants with d eletions in the N-terminal part. Deletions in the C-terminal domain re sulted in a complete loss of regulation, The altered response to chang es in osmolality led to severe consequences in the cellular adaption t o hyperosmotic stress. Whereas in the wild type, the steady state leve l of glycine betaine accumulation is maintained by activity regulation of the BetP system itself, in the mutant with BetP proteins carrying truncations in the C-terminal domain, the observed steady state betain e accumulation was found to be due to a kinetic balance of unregulated glycine betaine uptake by the modifed BetP and efflux via the mechano sensitive efflux channel for compatible solutes at the same time.