Analysis of the nucleotide sequence of the Streptomyces glaucescens tcmI genes provides key information about the enzymology of polyketide antibiotic biosynthesis

Abstract

Key information about the biosynthesis of polyketide metabolites had been uncovered by sequence analysis of the tetracenomycin C polyketide synthase genes (tcmI) from Streptomyces glaucescens GLA.0. The sequence data revealed the presence of three complete open reading frames (ORFs). ORF1 and ORF2 appear to be translationally coupled and would encode proteins containing 426 and 405 amino acids, respectively. The two deduced proteins are homologous to known β-ketoacyl synthases. ORF3 begins 70 nucleotides after the stop codon of ORF2 and would code for an 83 amino acid protein with a strong resemblance to known bacterial, animal and plant acyl-carrier proteins (ACP). The presence of an ACP gene within the tcm gene cluster suggests that different ACPs are used in fatty acid and polyketide biosynthesis in Streptomyces. We conclude from these data and earlier information that polyketide biosynthesis in S. glaucescens, and most likely in other bacteria, involves a multienzyme complex consisting of at least five types of enzymes: acylCoA transferases that load the acyl and 2-carboxyacyl precursors onto the ACP; a β-ketoacyl synthase that, along with the acylated ACP, forms the poly-β-ketoacyl intermediates; a poly-β-ketone cyclase that forms carbocyclic structures from the latter intermediates; a β-ketoacyl oxidoreductase that forms β-hydroxyacyl intermediates or reduces ketone groups in fully formed polyketides; and a thioesterase that releases the assembled polyketide from the enzyme.