CLONING AND CHARACTERIZATION OF TEK, THE GENE ENCODING THE MAJOR EXTRACELLULAR PROTEIN OF PSEUDOMONAS-SOLANACEARUM

Susceptible plants infected by Pseudomonas solanacearum usually wilt, largely due to extracellular proteins (EXPs) and the high-molecular-ma ss extracellular polysaccharide (EPS I) this pathogen produces. Circum stantial evidence suggested that a 28-kDa protein, the single most abu ndant EXP made by II solanacearum in culture, is associated with produ ction of EPS I, and thus might have a role in pathogenesis. The 28-kDa EXP was purified and, based on its N-terminal amino acid sequence, an oligonucleotide mixture was made and used as a hybridization probe to done the gene encoding it. DNA sequence analysis suggested that the c oding sequence for the 28-kDa EXP is within a gene, designated tek, th at encodes a 58-kDa membrane-associated precursor protein that is proc essed by signal peptidase II during export. Analysis of radiolabeled p olypeptides expressed from tek confirmed that it encodes a 58-kDa prec ursor protein, which is exported out of the cells as a 55-kDa preprote in and processed extracellularly to release the very basic 28-kDa EXP from its C terminus. The position, transcriptional direction, and regu lated expression of tek suggest that it is cotranscribed with xpsR, a gene essential for regulating biosynthesis of EPS I, and reinforces th e association of the 28-kDa EXP with virulence, However, since EI sola nacearum mutants lacking only the 28-kDa EXP produced wild-type amount s of EPS I and were fully virulent, the function of this protein remai ns unclear.