THE LPPC GENE OF STREPTOCOCCUS-EQUISIMILIS ENCODES A LIPOPROTEIN THATIS HOMOLOGOUS TO THE E(P4) OUTER-MEMBRANE PROTEIN FROM HAEMOPHILUS-INFLUENZAE

We report the cloning, sequencing, and analysis of a novel chromosomal gene of Streptococcus equisimilis strain H46A that codes for a membra ne lipoprotein, designated LppC. The lppC gene is located 3' adjacent to, and co-oriented with, the unrelated gapC gene that encodes the pre viously characterized glyceraldehyde-3-phosphate dehydrogenase. Sequen cing of lppC revealed an 855-bp open reading frame that predicted a 32 .4-kDa polypeptide possessing a potential lipoprotein signal sequence and modification site (VTGC). Signal sequence processing of LppC synth esized in the homologous host or expressed from plasmid pLPP2 in Esche richia coli was sensitive to globomycin, a selective inhibitor of lipo protein-specific signal peptidase II. Subcellular localization of LppC using polyclonal antibodies raised to the hexahistidyl-tagged protein proved LppC to be tightly associated with the cytoplasmic membrane of S. equisimilis and with the outer membrane of E. coli JM109 (pLPP2). Southern, Northern and Western analyses indicated that Ipl, was conser ved in S. pyogenes, and transcribed independently of gap as monocistro nic 0.9-kb mRNA from a sigma(70)-like consensus promoter. Database sea rches found homology of LppC to the hel gene-encoded outer membrane pr otein e (P4) from Haemophilus influenzae to which it exhibits 58% sequ ence similarity. However, unlike the hel gene, lppC was unable to comp lement hemA mutants of E. coli for growth on hemin as sole porphyrin s ource in aerobic conditions. Furthermore, neither the wild type nor an lppC insertion mutant of S. equisimilis could grow on hemin in iron-l imited medium. These results, together with findings indicating that S . equisimilis H46A had no absolute requirement for iron, led us to con clude that lppC, in contrast to hel, is not involved in hemin utilizat ion and has yet to be assigned a function.