First characterisation of a type III PKS gene cluster from cyanobacteria

Polyketide synthases (PKS) constitute a family of exceptional enzymes that biosynthesise secondary metabolites across many eukaryotic and prokaryotic phyla, including cyanobacteria. Mostly, researchers have focused on cyanobacterial type I PKS (PKSI) enzymes because of their involvement in toxin biosynthesis and their interesting multidomain structure. In contrast, research on type III PKS (PKSIII) enzymes, also a source of secondary metabolites and initially described for the biosynthesis of flavonoids, have been largely explored in plants.

To deepen our knowledge of PKSIII from cyanobacteria, several genomes were mined for putative type III PKS gene clusters. We identified a gene cluster encoding four genes, including a PKSIII in the genome of the cyanobacterium Raphidiopsis mediterranea, a filamentous marine and freshwater strain known also to produce the cytotoxin cylindrospermopsin. Using E. coli as a host, we have cloned and heterologously expressed both the complete gene cluster (RmTSC) and the single PKSIII enzyme (RmTS) from this cyanobacterium. Using several approaches (bioinformatics, calorimetry, circular dichroism, HPLC and NMR), we have characterised the product of RmTSC as well as the PKSIII enzyme.

We present a description of the PKSIII gene cluster, its product and the general characterisation of the PKSIII enzyme, its novel secondary structure, and the reactions catalysed by this pathway. To our knowledge, this is the first description of a cyanobacterial PKSIII. Due to their simple biochemistry and enzymology, compared to PKSI enzymes, the use of PKSIII to biosynthesise compounds opens a new avenue for the search and production of bioactive molecules from cyanobacteria.