ANALYSIS OF A STREPTOCOCCUS-PNEUMONIAE GENE ENCODING SIGNAL PEPTIDASE-I AND OVERPRODUCTION OF THE ENZYME

The spi gene of Streptococcus pneumoniae was cloned and its nucleotide sequence was determined. It encodes a protein of 204 amino acids that is homologous to bacterial signal peptidase I proteins. The S. pneumo niae protein contains all of the conserved amino acid sequence motifs previously identified in this enzyme from both prokaryotic and eukaryo tic sources. Sequence comparisons revealed several additional motifs c haracteristic of the enzyme. The cloned S. pneumoniae gene complemente d an Escherichia coli mutant defective in its leader peptidase gene. E xpression of the spi gene in S. pneumoniae appeared to be essential fo r viability. The cloned gene was shown to produce a polypeptide of app roximately 20 kDa. Overproduction of the S. pneumoniae spi gene in an E. coli expression system gave a native protein product, soluble in th e presence of a non-ionic detergent, which should be amenable to struc tural determination. (C) 1997 Elsevier Science B.V.