Lon protease of the α-proteobacterium Agrobacterium tumefaciens is required for normal growth, cellular morphology and full virulence

The ATP-dependent Lon (La) protease is ubiquitous in nature and regulates a diverse set of physiological responses in bacteria. In this paper a lon mutant of the alpha-proteobacterium Agrobacterium tumefaciens C58 has been characterized. Unlike lon mutants of Escherichia coli, the lon mutant of A. tumefaciens grows very slowly, is not filamentous and exhibits normal resistance to UV irradiation. The mutant retains motility and chemotaxis, produces apparently normal amounts of exopolysacchride, but displays severe defects in cell morphology, with 80 % of the mutant cells appearing Y-shaped. Lon protease of A. tumefaciens shares high homology with its counterparts in E. coli and in Sinorhizobium meliloti, and functionally complements an E. coli lon mutant for defects in morphology and RcsA-mediated regulation of capsular polysaccharide production. Mutations at sites of Lon(At) corresponding to the ATP-binding site and the active site serine of the E. coli Lon protease abolish complementation of phenotypes of the A. tumefaciens and E. coli lon mutants. The nucleotide sequence upstream of A. tumefaciens lon contains an element similar to the consensus sigma(32) heat-shock promoter of E. coli. Northern and Western blot analyses indicated that expression of lon is induced by elevated temperature, albeit to a much lower level than that of groEL. The lon mutant is highly attenuated for virulence, suggesting that Lon may be required for the proper expression, assembly or function of the VirB/D4-mediated T-DNA transfer system.