Identification by Heterologous Expression and Gene Disruption of VisA as l -Lysine 2-Aminotransferase Essential for Virginiamycin S Biosynthesis in Streptomyces virginiae

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Abstract
The visA gene of Streptomyces virginiae has been thought to be a part of the virginiamycin S (VS) biosynthetic gene cluster based on its location in the middle of genes that encode enzymes highly similar to those participating in the biosynthesis of streptogramin-type antibiotics. Heterologous expression of the visA gene was achieved in Escherichia coli by an N-terminal fusion with thioredoxin (TrxA), and the intact recombinant VisA protein (rVisA) was purified after cleavage with enterokinase to remove the TrxA moiety. The purified rVisA showed clear l -lysine 2-aminotransferase activity with an optimum pH of around 8.0 and an optimum temperature at 35°C, with 2-oxohexanoate as the best amino acceptor, indicating that VisA converts l -lysine into Δ 1 -piperidine 2-carboxylic acid. A visA deletion mutant of S. virginiae was created by homologous recombination, and the in vivo function of the visA gene was studied by phenotypic comparison between the wild type and the visA deletion mutant. No differences in growth in liquid media or in morphological behavior on solid media were observed, indicating that visA is not involved in primary metabolism or morphological differentiation. However, the visA mutant failed to produce VS while maintaining the production of virginiamycin M 1 at a level comparable to that of the parental wild-type strain, demonstrating that visA is essential to VS biosynthesis. These results, together with the observed recovery of the defect in VS production by the external addition of 3-hydroxypicolinic acid (3-HPA), a starter molecule in VS biosynthesis, suggest that VisA is the first enzyme of the VS biosynthetic pathway and that it supplies 3-HPA from l -lysine.