TheSip(Sli)Gene ofStreptomyces LividansTK24 Specifies an Unusual Signal Peptidase with a Putative C-Terminal Transmembrane Anchor

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Abstract
Type I signal peptidases (SPases) are a widespread family of enzymes which remove signal peptides from proteins translocated across cellular membranes. Here, we report the first isolation of a gene coding for type I signal peptidase of Streptomyces, denoted Sip(Sli). The stp(sli)gene specifies a protein of 291 amino acids. Thus Sip(Sli) is much larger (approximately 100 amino acids) than other known SPases of Gram-positive bacteria and resembles SPases of Gram-negative bacteria, showing the highest degree of similarity to an SPase of the cyanobac-terium Phormidium laminosum. Sip(Sli) contains conserved serine and lysine residues, which are believed to be required for the catalytic activity. Similar to other known SPases from Gram-positive bacteria, Sip(Sli) seems to have only one N-terminal transmembrane anchor. In addition, Sip(Sli) seems to contain a second transmembrane anchor at the C-terminus, which is an unusual feature for type I signal peptidases.