Regulation of an anthranilate synthase gene in Streptomyces venezuelae by a trp attenuator

Abstract

The nucleotide sequence of a 2·4 kb BamHI--Sall fragment of Streptomyces venezuelae ISP5230 DNA that complements trpE and trpG mutations in Escherichia coli contains two ORFs. The larger of these (ORF2) encodes a 624 amino acid sequence similar to the overall sequence of the two subunits of anthranilate synthase. The two-thirds nearest the amino terminus resembles the aminase subunit; the remaining one-third resembles the glutamine amidotransferase subunit. Upstream of ORF2 is a small ORF encoding 18 amino acids that include three adjacent Trp residues; in addition the ORF contains inverted repeats with sequence and positional similarity to the products of attenuator (trpL) regions that regulate tryptophan biosynthesis in other bacteria. In cultures of a trpC mutant of S. venezuelae, increasing the concentration of exogenous tryptophan decreased the formation of anthranilate synthase; similar evidence of endproduct repression was obtained in a trpCER mutant of E. coli transformed with a vector containing the cloned DNA fragment from S. venezuelae. The anthranilate synthase activity in S. venezuelae cell extracts was inhibited by tryptophan, although only at high concentrations of the amino acid. A two-base deletion introduced into the cloned S. venezuelae DNA fragment prevented complementation of a trpE mutation in E. coli. However, S. venezuelae transformants in which the two-base deletion had been introduced by replacement of homologous chromosomal DNA did not exhibit a Trp- phenotype. The result implies that S. venezuelae has one or more additional genes for anthranilate synthase. In alignments with anthranilate synthase genes from other organisms, ORF2 from S. venezuelae most closely resembled genes for phenazine biosynthesis in Pseudomonas. The results bear on the function of the gene in S. venezuelae.