CHARACTERIZATION OF THE FIMA GENE ENCODING BUNDLE-FORMING FIMBRIAE OFTHE PLANT PATHOGEN XANTHOMONAS-CAMPESTRIS PV VESICATORIA

The fimA gene of Xanthomonas campestris pv, vesicatoria was identified and characterized. A 20-mer degen- erate oligonucleotide complementar y to the N-terminal amino acid sequence of the purified 15.5-kDa fimbr illin was used to locate fimA on a 2.6-kb Sa/I fragment of the X. camp estris pv, vesicatoria 3240 genome. The nucleotide sequence of a 1.4-k b fragment containing the fimA region revealed two open reading frames predicting highly homologous proteins FimA and FimB. FimA, which was composed of 136 amino acids and had a calculated molecular weight of 1 4,302, showed high sequence identity to the type IV fimbrillin precurs ors.fimB predicted a protein product of 135 amino acids and a molecula r weight of 13,854. The open reading frame for fimB contained near the 5' end a palindromic sequence with a terminator loop potential, and t he expression level of JimB in vitro and in Xanthomonas was considerab ly lower than that ofJimA. We detected an efficiently transcribed fimA -specific mRNA of 600 bases as well as two weakly expressed, longer mR NA species that reacted with both fimA and fimB. A homolog of fimA but not of fimB? was detected by Southern hybridization in strains of X. campestris pv, vesicatoria, campestris, begoniae, translucens, and gra minis. A fimA::Omega mutant of strain 3240 was not significantly reduc ed in virulence or adhesiveness to tomato leaves. However, the fimA mu tant was dramatically reduced in cell aggregation in laboratory cultur es and on infected tomato leaves, The fimA mutant strain also exhibite d decreased tolerance to UV light.