Analysis of the genes flanking xabB: a methyltransferase gene is involved in albicidin biosynthesis in Xanthomonas albilineans

Transposon mutagenesis and complementation studies previously identified a gene (xabB) for a large (526 kDa) polyketide-peptide synthase required for biosynthesis of albicidin antibiotics and phytotoxins in the sugarcane leaf scald pathogen Xanthomonas albilineans. A cistron immediately downstream f rom xabB encodes a polypeptide of 343 aa containing three conserved motifs characteristic of a family of S-adenosyl-L-methionine (SAM)-dependent O-met hyltransferases. Insertional mutagenesis and complementation indicate that the product of this cistron (designated xabC) is essential for albicidin pr oduction, and that there is no other required downstream cistron. The xab p romoter region is bidirectional, and insertional mutagenesis of the first o pen reading frame (ORF) in the divergent gene also blocks albicidin biosynt hesis. This divergent ORF (designated thp) encodes a protein of 239 aa disp laying high similarity to several IS21-like transposition helper proteins. The thp cistron is not located in a recognizable transposon, and is probabl y a remnant from a past transposition event that may have contributed to th e development of the albicidin biosynthetic gene cluster. Failure of 'in tr ans' complementation of rhp indicates that a downstream cistron transcribed with thp is required for albicidin biosynthesis. (C) 2000 Elsevier Science B.V. All rights reserved.