THE REGULATORY VIRA-PROTEIN OF AGROBACTERIUM-TUMEFACIENS DOES NOT FUNCTION AT ELEVATED-TEMPERATURES

Previous studies have shown that Agrobacterium tumefaciens causes tumo rs on plants only at temperatures below 32-degrees-C, and virulence ge ne expression is specifically inhibited at temperatures above 32-degre es-C. We show here that this effect persists even when the virA and vi rG loci are expressed under the control of a lac promoter whose activi ty is temperature independent. This finding suggests that one or more steps in the signal transduction process mediated by the VirA and VirG proteins are temperature sensitive. Both the autophosphorylation of V irA and the subsequent transfer of phosphate to VirG are shown to be s ensitive to high temperatures (>32-degrees-C), and this correlates wit h the reduced vir gene expression observed at these temperatures. At t emperatures of 32-degrees-C and higher, the VirA molecule undergoes a reversible inactivation while the VirG molecule is not affected. vir g ene induction is temperature sensitive in an acetosyringone-independen t virA mutant background but not in a virG constitutive mutant which i s virA and acetosyringone independent. These observations all support the notion that the VirA protein is responsible for the thermosensitiv ity of vir gene expression. However, an Agrobacterium strain containin g a constitutive virG locus still cannot cause tumors on Kalanchoe pla nts at 32-degrees-C. This strain induces normal-size tumors at tempera tures up to 30-degrees-C, whereas the wild-type Agrobacterium strain p roduces almost no tumors at 30-degrees-C. These results suggest that a t temperatures above 32-degrees-C, the plant becomes more resistant to infection by A. tumefaciens and/or functions of some other vir gene p roducts are lost in spite of their normal levels of expression.