A luxR homolog, aviR, in Agrobacterium vitis is associated with induction of necrosis on grape and a hypersensitive response on tobacco

A Tn5 mutant of Agrobacterium vitis F2/5 (M1154) differs from the wild-type strain in that it has lost its abilities to cause necrosis on grape and a hypersensitive-like response (HR) on tobacco. The Tn5 insertion occurred in an open reading frame (ORF) aviR that is homologous to genes encoding the LuxR family of transcriptional regulators, thereby suggesting that the HR and necrosis are regulated by a quorum-sensing system. Fewer N-acyl-homoserine lactone autoinducers were detected in extracts from M1154 compared with extracts from F2/5 and from aviR-complemented M1154. The complemented mutant regained full ability to cause grape necrosis and HR. Eighteen ORFs located on a 36.6-kb insert in cosmid clone CPB221, which includes aviR, were sequenced and aligned with homologous genes from A. tumefaciens C58 and Sinorhizobium meliloti Rm1021. The order of several clustered genes is conserved among the bacteria; however, rearrangements are also apparent. Reverse transcriptase-pollymerase chain reaction analysis indicated that ORF2 and ORF14 may be regulated by an aviR-encoded transcriptional regulator. Single site-directed mutations in each of the ORFs, however, had no effect on expression of HR or necrosis as compared with the wild-type parent.