AMINO-ACID AND DNA-SEQUENCES OF AN EXTRACELLULAR BASIC PROTEASE OF DICHELOBACTER-NODOSUS SHOW THAT IT IS A MEMBER OF THE SUBTILISIN FAMILY OF PROTEASES

A DNA fragment encoding an extracellular basic protease (pI almost-equal-to 9.5) from Dichelobacter nodosus, a Gram-negative obligate anaerobe and the causative agent of ovine footrot, has been cloned and expressed in Escherichia coli and sequenced. E. coli harbouring a plasmid with a 3-kb DNA fragment containing the D. nodosus basic-protease gene exhibited proteolytic activity when tested on skim-milk plates. The sequence of the native basic protease isolated from D. nodosus was also determined by direct amino acid sequencing. Comparison of the deduced sequence of the primary translation product (603 residues) and that of the native protease (344 residues) indicates that the protease is synthesized as a precursor molecule, containing a signal peptide (21 residues), a 111 amino acid pro-peptide and a 127 residue C-terminal extension which is subsequently processed to the mature active form. Comparison of the D. nodosus basic protease sequence with that of other serine proteases showed that it is related to the subtilisin family of proteases with strong conservation of sequence identity around the catalytic site residues. A remarkable similarity in structure was found to the serine protease of Xanthomonas campestris, a plant pathogen, with respect to the length of the precursor segments, conservation of disulfide bridges and approximately 50% sequence identity of the mature proteases.