DIFFERENTIAL EXPRESSION OF CONSERVED PROTEASE GENES IN CRUCIFER-ATTACKING PATHOVARS OF XANTHOMONAS-CAMPESTRIS

Strains of Xanthomonas campestris pathovars armoraciae and raphani, wh ich cause leaf spotting diseases in brassicas, produce a major extrace llular protease in liquid culture which was partially purified. The pr otease (PRT 3) was a zinc-requiring metalloenzyme and was readily dist inguishable from the two previously characterized proteases (PRT 1 and PRT 2) of X. campestris pv. campestris by the pattern of degradation of 13-casein and sensitivity to inhibitors. PRT 3 was produced at a lo w level in the vascular brassica pathogen X. campestris pv. campestris (five strains tested), in which PRT 1 and PRT 2 predominate. In contr ast, expression of PRT 1, a serine protease, could not be detected in the six tested strains of the leaf spotting mesophyll pathogens. Howev er, all these strains had DNA fragments which hybridized to a prtA pro be and which probably carry a functional prtA (the structural gene for PRT 1). The structural gene for PRT 3 (prtC) was cloned by screening a genomic library of X. campestris pv. raphani in a protease-deficient X. campestris pv. campestris strain. Subcloning and Tn5 mutagenesis l ocated the structural gene to 1.2 kb of DNA. DNA fragments which hybri dized to the structural gene were found in all strains of the crucifer -attacking X. campestris pathovars tested as well as in a number of ot her pathovars. Experiments in which the pattern of protease production of the pathovars was manipulated by introduction of cloned genes into heterologous pathovars suggested that no determinative relationship e xists between the pattern of protease gene expression and the (vascula r or mesophyllic) mode of pathogenesis.