Tryptophan (Trp) halogenases are found in various bacteria and play an important role in natural product biosynthesis. Analysis of the genome of Streptomyces toxytricini NRRL 15443 revealed an ORF, stth, encoding a putative Trp halogenase within a non-ribosomal peptide synthetase gene cluster. This gene was cloned into pET28a and functionally over-expressed in Escherichia coli. The enzyme halogenated both L-and D-Trp to yield the corresponding 6-chlorinated derivatives. The optimum activity was at 40 degrees C and pH 6 giving k(cat)/K-M value of STTH of 72,000 min(-1) M-1. The enzyme also used bromide to yield 6-bromo-Trp.