Stability of the Agrobacterium tumefaciens VirB10 protein is modulated by growth temperature and periplasmic osmoadaption

Export of oncogenic T-DNA from the phytopathogen Agrobacterium tumefaciens is mediated by the products of the virB operon. It has recently been report ed (K. J. Fullner and E. W. Nester, J. Bacteriol. 178:1498-1504, 1996) that DNA transfer does not occur at elevated temperatures; these observations c orrelate well with much earlier studies on the temperature sensitivity of c rown gall tumor development on plants. In testing the hypothesis that this loss of DNA movement reflects a defect in assembly or maintenance of a stab le DNA transfer machinery at high temperature, we have found that steady-st ate levels of VirB10 are sensitive to growth temperature while levels of se veral other VirB proteins are considerably less affected. This temperature- dependent failure to accumulate VirB10 is exacerbated in an attachment-defi cient mutant strain (chvB) which exhibits pleiotropic defects in periplasmi c osmoadaption, and virulence of a chvB mutant can be partially restored by lowering the temperature at which the bacteria and the plant tissue are co cultivated. Furthermore, the stability of VirB10 is diminished in cells lac king functional VirB9, but only under conditions of low osmolarity. We prop ose that newly synthesized VirB10 is inherently labile in the presence of a large osmotic gradient across the inner membrane and is rapidly degraded u nless it is stabilized by VirB9-dependent assembly into oligomeric complexe s. The possibility that VirB10-containing complexes are not assembled prope rly at elevated temperatures suggests an explanation for the decades-old ob servation that tumor formation is exquisitely sensitive to ambient temperat ure.