Cyclic heptapeptide microcystin biosynthesis requires the glutamate racemase gene

It was demonstrated previously that the operon consisting of the nonribosomal peptide synthetase (NRPS) gene coupled with the polyketide synthase (PKS) gene involved in cyclic heptapeptide microcystin synthesis includes two different D-amino acid synthetase genes, an epimerization domain at the 3' end of module 2, and the racemase gene mcyF, To determine the role of mcyF in microcystin synthesis, gene-disruption and complementation analyses were carried out. Insertional mutagenesis in the mcyF gene, generated by homologous recombination, abolished only microcystin synthesis, but did not influence cell growth, Furthermore, McyF supported D-Glu-independent growth of a strain of Escherichia coli defective in D-Glu synthesis, It is concluded that mcyF is the glutamic acid racemase gene involved in the synthesis of D-Glu residues in the microcystin molecule. This is the first report of the racemase in prokaryotic NRPS.