SMb20651 is another acyl carrier protein from Sinorhizobium meliloti

Abstract

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 ofSinorhizobium meliloti1021 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 inEscherichia coli,[3H]β-alanine, a biosynthetic building block of 4′-phosphopantetheine, was incorporated into the proteinin vivo. The purified SMb20651 was modified with 4′-phosphopantetheine in the presence ofS. melilotiholo-ACP synthase (AcpS). Also, holo-SMb20651 was modifiedin vitrowith a malonyl group by malonyl CoA-ACP transacylase. InE. 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 inS. melilotias holo-SMb20651 and acyl-SMb20651.