AN OUTER-MEMBRANE PORIN INDUCIBLE BY SHORT-CHAIN AMIDES AND UREA IN THE METHYLOTROPHIC BACTERIUM METHYLOPHILUS-METHYLOTROPHUS

The fmdA and fmdB genes encoding formamidase and a putative regulatory protein, respectively, from the methylotrophic bacterium Methylophilu s methylotrophus were recloned with additional flanking DNA (pSW1). fm dC, encoding a weakly hydrophilic protein containing an N-terminal sig nal sequence, was identified upstream of fmdAB. The derived amino acid sequence of mature FmdC (M-r 39204) showed that it was rich in beta-s heet and aromatic amino acids, and exhibited significant similarities to several outer-membrane porins from other bacteria, Cell fractionati on studies showed that the protein was located in the outer membrane. Mature FmdC was purified and shown to consist of a single type of subu nit (M-r 40 000) with the predicted N-terminal amino acid sequence (GA TISF-). SDS-PAGE and Western blotting of cells grown in continuous cul ture under various conditions showed that mature FmdC was induced by f ormamide, acetamide and urea, repressed by excess ammonia, and over-ex pressed during prolonged growth under formamide limitation, It is conc luded that mature FmdC is a porin involved in the transport of short-c hain amides and urea through the outer membrane of M. methylotrophus u nder conditions where these nitrogen sources are present at very low c oncentration.