The Sip(Sli) gene of Streptomyces lividans TK24 specifies an unusual signal peptidase with a putative C-terminal transmembrane anchor

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 pe ptidase of Streptomyces, denoted Sip(Sli) The sip(sli) gone specifies a pro tein of 291 amino acids. Thus Slp(Sli) is much larger (approximately 100 am ino 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 cyanobacterium Phormidium laminosum. Sip(Sli) contains c onserved serine and lysine residues, which are believed to be required for the catalytic activity. Similar to other known SPases from Gram-positive ba cteria, 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.