A giant type I polyketide synthase participates in zygospore maturation in Chlamydomonas reinhardtii.

Polyketide synthases (PKSs) occur in many bacteria, fungi and plants. They are highly versatile enzymes involved in the biosynthesis of a large variety of compounds including antimicrobial agents, polymers associated with bacterial cell walls and plant pigments. While harmful algae are known to produce polyketide toxins, sequences of the genomes of non-toxic algae, including those of many green algal species, have surprisingly revealed the presence of genes encoding type I PKSs. The genome of the model alga Chlamydomonas reinhardtii (Chlorophyta) contains a single type I PKS gene, designated PKS1 (Cre10.g449750), which encodes a giant PKS with a predicted mass of 2.3 MDa. Here, we show that PKS1 is induced in 2-day-old zygotes and is required for their development into zygospores, the dormant stage of the zygote. Wild-type zygospores contain knob-like structures (~50 nm diameter) that form at the cell surface and develop a central cell wall layer; both of these structures are absent from homozygous pks1 mutants. Additionally, in contrast to wild-type zygotes, chlorophyll degradation is delayed in homozygous pks1 mutant zygotes, indicating a disruption in zygospore development. In agreement with the role of the PKS in the formation of the highly resistant zygospore wall, mutant zygotes have lost the formidable desiccation tolerance of wild-type zygotes. Together, our results represent functional analyses of a PKS mutant in a photosynthetic eukaryotic microorganism, revealing a central function for polyketides in the sexual cycle and survival under stressful environmental conditions.

Multimodular type I polyketide synthases in algae evolve by module duplications and displacement of AT domains in trans.

BACKGROUND: Polyketide synthase (PKS) catalyzes the biosynthesis of polyketides, which are structurally and functionally diverse natural products in microorganisms and plants. Here, we have analyzed available full genome sequences of microscopic and macroscopic algae for the presence of type I PKS genes. RESULTS: Type I PKS genes are present in 15 of 32 analyzed algal species. In chlorophytes, large proteins in the MDa range are predicted in most sequenced species, and PKSs with free-standing acyltransferase domains (trans-AT PKSs) predominate. In a phylogenetic tree, PKS sequences from different algal phyla form clades that are distinct from PKSs from other organisms such as non-photosynthetic protists or cyanobacteria. However, intermixing is observed in some cases, for example polyunsaturated fatty acid (PUFA) and glycolipid synthases of various origins. Close relationships between type I PKS modules from different species or between modules within the same multimodular enzyme were identified, suggesting module duplications during evolution of algal PKSs. In contrast to type I PKSs, nonribosomal peptide synthetases (NRPSs) are relatively rare in algae (occurrence in 7 of 32 species). CONCLUSIONS: Our phylogenetic analysis of type I PKSs in algae supports an evolutionary scenario whereby integrated AT domains were displaced to yield trans-AT PKSs. Together with module duplications, the displacement of AT domains may constitute a major mechanism of PKS evolution in algae. This study advances our understanding of the diversity of eukaryotic PKSs and their evolutionary trajectories.