A gene for superoxide dismutase from Xanthomonas campestris pv. campestris and its expression during bacterial-plant...

A recombinant plasmid selected from a library of Xanthomonas campestris pv. campestris genomic DNA by functional complementation of a superoxide dismutase (SOD)-deficient strain of Escherichia coli contained a gene encoding the major SOD activity of X. campestris pv. campestris. Inhibition and renaturation studies suggested that manganese was the metal cofactor for this SOD. Examination of the nucleotide sequence of an active subclone revealed a 612-bp open reading frame that encodes a protein with high amino acid sequence homology to a range of SOD enzymes. The sod gene was mutagenized with Tn5-lacZ. None of the insertions that abolished SOD-conferring activity were in the correct orientation for lacZ expression. Repeated attempts to introduce these insertions into the chromosome of X. campestris pv. campestris were unsuccessful and it was concluded that the sod gene may be essential for viability. In order to monitor the expression of the sod gene, a sod-gus transcriptional fusion was constructed. Expression of the sod gene varied according to the growth stage of the organism in culture. In planta, the sod gene was induced within 3 to 4 h of inoculation, with similar kinetics during compatible and incompatible interactions with turnip and pepper, respectively.