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SMb20651 is another acyl carrier protein from Sinorhizobium meliloti

¹ Programa de Ecología Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apdo. Postal 565-A, Cuernavaca, Morelos C. P. 62251, Mexico
² Programa de Genómica Funcional de Procariontes, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apdo. Postal 565-A, Cuernavaca, Morelos C. P. 62251, Mexico

Correspondence
Isabel M. López-Lara
isabel{at}ccg.unam.mx

Acyl carrier proteins (ACPs) are small acidic proteins that carry growing acyl chains during fatty acid or polyketide synthesis. In rhizobia, there are four different and well-characterized ACPs: AcpP, NodF, AcpXL and RkpF. The genome sequence of Sinorhizobium meliloti 1021 reveals two additional ORFs that possibly encode additional ACPs. One of these, smb20651, is located on the plasmid pSymB as part of an operon. The genes of the operon encode a putative asparagine synthetase (AsnB), the predicted ACP (SMb20651), a putative long-chain fatty acyl-CoA ligase (SMb20650) and a putative ammonium-dependent NAD⁺ synthetase (NadE1). When SMb20651 was overexpressed in Escherichia coli, [³H]β-alanine, a biosynthetic building block of 4'-phosphopantetheine, was incorporated into the protein in vivo. The purified SMb20651 was modified with 4'-phosphopantetheine in the presence of S. meliloti holo-ACP synthase (AcpS). Also, holo-SMb20651 was modified in vitro with a malonyl group by malonyl CoA-ACP transacylase. In E. coli, coexpression of SMb20651 together with other proteins such as AcpS and SMb20650 led to the formation of additional forms of SMb20651. In this bacterium, acylation of SMb20651 with C12 : 0 or C18 : 0 fatty acids was detected, demonstrating that this protein is involved in fatty acid biosynthesis or transfer. Expression of SMb20651 was detected in S. meliloti as holo-SMb20651 and acyl-SMb20651.

Two supplementary figures, showing results of RT-PCR analysis demonstrating that asnB, smb20651, smb20650 and nadE1 are encoded in an operon, the scheme of asnB, smb20651, smb20650 and nadE1 arrangement, and a multiple sequence alignment of the S. meliloti acyl carrier proteins and selected SMb20651 homologues, are available with the online version of this paper.