The Streptomyces venezuelae pikAV gene contains a transcription unit essential for expression of enzymes involved in glycosylation of narbonolide and10-deoxymethynolide

In Streptomyces venezuelae, four polyketide synthase (PKS) polypeptides enc oded by pikAI-pikAIV are used to generate 10 and 12-membered macrocyclic st ructures, narbonolide and 10-deoxymethynolide. Sequence analysis suggests t hese genes are translationally coupled with downstream genes, pikAV (encodi ng a type II thioesterase), desVIII-desVI (encoding enzymes responsible for production of the final glycosylated products pikromycin, narbomycin, meth ymycin and neomethymycin) and desR (a resistance gene). Type II thioesteras es have been suggested to have an editing function in polyketide biosynthes is and deletion of the corresponding genes often leads to decreased levels of polyketide production. Surprisingly an in-frame deletion of 687 bp of th e 843 bp pikAV ORF led to a strain SC1022 that produced normal yields of po lyketide products, but only in the aglycone form. Plasmid-based expression of the desVIII-VI and desR in the SC1022 strain completely restored product ion of glycosylated products, despite the absence of a functional pikAV gen e product. Under these conditions the PikAV TEII therefore does not play an important role in polyketide biosynthesis, and its function remains an eni gma. These observations also demonstrate that the region of pikAV DNA delet ed in strain SC1022 contains a transcription unit essential for expression of the des genes. A sequence alignment of PikAV with members of the highly conserved type II thioesterases revealed a short divergent region at the ca rboxy terminus, suggesting a region of pikAV that might contain such a tran scription unit. DNA containing this region of pikAV was shown to be able to increase plasmid-based expression of both crotonyl CoA reductase gene (ccr ) and the erythromycin resistance gene (ermE) in S. venezuelae. (C) 2001 El sevier Science B.V. All rights reserved.