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The Endo-β-1,4-glucanase CelA of Clavibacter michiganensis subsp. michiganensis Is a Pathogenicity Determinant Required for Induction of Bacterial Wilt of Tomato

The phytopathogenic bacterium Clavibacter michiganensis subsp. michiganensis NCPPB382, which causes bacterial wilt and canker of tomato, harbors two plasmids, pCM1 (27.35 kb) and pCM2 (72 kb), encoding genes involved in virulence (D. Meletzus, A. Bermpohl, J. Dreier, and R. Eichenlaub, 1993, J. Bacteriol. 175:2131--2136; J. Dreier, D. Meletzus, and R. Eichenlaub, 1997, Mol. Plant-Microbe Interact. 10:195--206). The region of pCM1 carrying the endoglucanase gene celA was mapped by deletion analysis and complementation. RNA hybridization identified a 2.4-knt (kilonucleotide) transcript of the celA structural gene and the transcriptional initiation site was mapped. The celA gene encodes CelA, a protein of 78 kDa (746 amino acids) with similarity to endo-β-1,4-glucanases of family A₁ cellulases. CelA has a three-domain structure with a catalytic domain, a type IIa-like cellulose-binding domain, and a C-terminal domain. We present evidence that CelA plays a major role in pathogenicity, since wilt induction capability is obtained by endoglucanase expression in plasmid-free, nonvirulent strains and by complementation of the CelA^- gene-replacement mutant CMM-H4 with the wild-type celA gene.