A Novel β/ε Subunit Combination Expands the Tri-Subunit Acyl-CoA Carboxylase Repertoire in Streptomyces coelicolor

Acyl-CoA carboxylase (YCC) complexes generate essential starter and extender units for fatty acid and polyketide biosynthesis in Actinobacteria. In Streptomyces coelicolor, two tri-subunit YCC complexes, acetyl-CoA carboxylase (ACC) and propionyl-CoA carboxylase (PCC), have been characterized. However, comparative genomic analyses indicate that β/ε subunits are more diversified than currently appreciated. Here, we identify a previously unrecognized β/ε pair, AccB2 and AccE2, and demonstrate that they assemble with the canonical α subunit to form a functional YCC complex. Both genes are transcribed in vivo, and co-immunoprecipitation (Co-IP) reveals association with AccA1 and AccA2, with AccE2 showing stronger relative association with AccA1-containing pull-downs. In vitro reconstitution confirms carboxylation activity toward acetyl-CoA, propionyl-CoA, and butyryl-CoA, which is strongly dependent on AccE2. These findings expand the YCC repertoire in S. coelicolor and support a modular assembly model in which alternative β/ε combinations contribute to functional diversification of YCC complexes.