Osmosensing and osmosignaling in Corynebacterium glutamicum.

The Gram-positive soil bacterium Corynebacterium glutamicum is used in microbial biotechnology for the large-scale production of amino acids, e.g., L: -glutamate and L: -lysine. We have studied the response of this organism to hyperosmotic challenge at the level of both transcription and protein activity. Two systems responding to hyperosmotic stress in C. glutamicum are reviewed here, the two component system MtrAB and the glycine-betaine uptake system BetP. The osmosensory two-component system consists of the membrane-bound histidine kinase MtrB and the soluble response regulator MtrA. MtrB was shown to perceive a so far unknown physical stimulus related to hyperosmotic stress via the cytoplasmically oriented phosphorylation domain, and to transduce the signal to the DNA via MtrA. The secondary active transporter BetP takes up betaine in cotransport with two Na(+) ions. BetP responds to hyperosmotic stress by increased transcription mediated via MtrAB signaling, and by instant activation of transport. In the mechanism of BetP activation, the C-terminal, regulatory domain of BetP, the cytoplasmic concentration of K(+), and negative membrane surface charges are involved. The molecular mechanism of the activation process is discussed in relation to the recently published X-ray structure of BetP.