<jats:title>ABSTRACT</jats:title> <jats:p> The biosynthetic gene cluster (12.3 kb) of mersacidin, a lanthionine-containing antimicrobial peptide, is located on the chromosome of the producer, <jats:italic>Bacillus</jats:italic> sp. strain HIL Y-85,54728 in a region that corresponds to 348° on the chromosome of <jats:italic>Bacillus subtilis</jats:italic> 168. It consists of 10 open reading frames and contains, in addition to the previously described mersacidin structural gene <jats:italic>mrsA</jats:italic> (G. Bierbaum, H. Brötz, K.-P. Koller, and H.-G. Sahl, FEMS Microbiol. Lett. 127:121–126, 1995), two genes, <jats:italic>mrsM</jats:italic> and <jats:italic>mrsD</jats:italic> , coding for enzymes involved in posttranslational modification of the prepeptide; one gene, <jats:italic>mrsT</jats:italic> , coding for a transporter with an associated protease domain; and three genes, <jats:italic>mrsF</jats:italic> , <jats:italic>mrsG</jats:italic> , and <jats:italic>mrsE</jats:italic> , encoding a group B ABC transporter that could be involved in producer self-protection. Additionally, three regulatory genes are part of the gene cluster, i.e., <jats:italic>mrsR2</jats:italic> and <jats:italic>mrsK2</jats:italic> , which encode a two-component regulatory system which seems to be necessary for the transcription of the <jats:italic>mrsFGE</jats:italic> operon, and <jats:italic>mrsR1</jats:italic> , which encodes a protein with similarity to response regulators. Transcription of <jats:italic>mrsA</jats:italic> sets in at early stationary phase (between 8 and 16 h of culture).