Functional analysis of a putative type III polyketide synthase from deep-sea sediment metagenome

Type III polyketide synthases (type III PKSs) are single homodimeric enzymes that produce diverse products such as phloroglucinol, pyrones, resorcinols and chalcones which are biotechnologically important molecules. In an attempt to identify new type III PKS from extreme environments, the deep-sea sediment metagenome from Bay of Bengal was screened for type III PKS genes. BLASTX analyses of Nanopore sequence derived metagenome with the in-house created PKS database revealed a full length type III PKS from a 5 kb fragment. The annotated full length type III PKS, S9PKS showed 25-30 % sequence identity towards previously characterized enzymes. To functionally characterize the gene, it was synthesized, cloned into pET28a and pColdI vectors under T7 and csp promoters, respectively, and expressed in Escherichia coli Rosetta(DE3) pLysS. The optimized PKS (OptiPKS) was expressed as inclusion bodies under both promoters. The inclusion bodies were successfully solubilised using low concentration of urea, refolded and purified using Ni-NTA Agarose resin. The purified OptiPKS was tested for functionality using fatty acyl-CoA substrates at various temperatures. High performance liquid chromatography (HPLC) analyses revealed that OptiPKS produced tri and tetraketide pyrones using C₄ to C₁₀ acyl-CoA starter substrates. Further characterization and mutation of the enzyme would reveal its functional significance. Thus, the study could be a lead for the annotation and functional characterization of putative type III PKS from environmental metagenome data.