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J. Biol. Chem., Vol. 280, Issue 8, 6399-6408, February 25, 2005

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Tandemly Duplicated Acyl Carrier Proteins, Which Increase Polyketide Antibiotic Production, Can Apparently Function Either in Parallel or in Series^*

Ayesha S. Rahman, Joanne Hothersall, John Crosby¶, Thomas J. Simpson¶, and Christopher M. Thomas||

From the School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom and the ^¶School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS United Kingdom

Polyketide biosynthesis involves the addition of subunits commonly derived from malonate or methylmalonate to a starter unit such as acetate. Type I polyketide synthases are multifunctional polypeptides that contain one or more modules, each of which normally contains all the enzymatic domains for a single round of extension and modification of the polyketide backbone. Acyl carrier proteins (ACP(s)) hold the extender unit to which the starter or growing chain is added. Normally there is one ACP for each ketosynthase module. However, there are an increasing number of known examples of tandemly repeated ACP domains, whose function is as yet unknown. For the doublet and triplet ACP domains in the biosynthetic pathway for the antibiotic mupirocin from Pseudomonas fluorescens NCIMB10586 we have inactivated ACP domains by inframe deletion and amino acid substitution of the active site serine. By deletion analysis each individual ACP from a cluster can provide a basic but reduced activity for the pathway. In the doublet cluster, substitution analysis indicates that the pathway may follow two parallel routes, one via each of the ACPs, thus increasing overall pathway flow. In the triplet cluster, substitution in ACP5 blocked the pathway. Thus ACP5 appears to be arranged "in series" to ACP6 and ACP7. Thus although both the doublet and triplet clusters increase antibiotic production, the mechanisms by which they do this appear to be different and depend specifically on the biosynthetic stage involved. The function of some ACPs may be determined by their location in the protein rather than absolute enzymic activity.

Received for publication, August 26, 2004 , and in revised form, October 27, 2004.

^* This work was funded in part by Biotechnology and Biological Sciences Research Center Project Grant P15257 (which also supported J. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Supported by a studentship from the Darwin Trust of Edinburgh.

^|| To whom correspondence should be addressed. Tel.: 44-121-414-5903; Fax: 44-121-414-5925; E-mail: c.m.thomas{at}bham.ac.uk.

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