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. pneumoniae protein contains all of the conserved amino acid sequence motifs previously identified in this enzyme from both prokaryotic and eukaryotic sources. Sequence comparisons revealed several additional motifs characteristic of the enzyme. The cloned S. pneumoniae gene complemented an Escherichia coli mutant defective in its leader peptidase gene. Expression of the spi gene in S. pneumoniae appeared to be essential for viability. The cloned gene was shown to produce a polypeptide of approximately 20 kDa. Overproduction of the S. pneumoniae spi gene in an E. coli expression system gave a native protein product, soluble in the presence of a non-ionic detergent, which should be amenable to structural determination.