Many paths to chlorophyll: the evolution of protochlorophyllide oxidoreductases in the algae

Abstract

Cyanobacteria, eukaryotic algae and some land plants catalyze the second to last step of chlorophyll a synthesis with either of two non-homologous enzymes: the light-independent (LIPOR) and light-dependent (POR) protochlorophyllide oxidoreductases. Both taxa with and taxa without LIPOR genes are reported for the chlorophyte, rhodophyte, cryptophyte, heterokont, and chromerid algal lineages. Haptophyte and chlorarachniophyte algal taxa appear to lack LIPOR genes. In contrast, genes encoding POR were found in all algal nuclear genomes sequenced to date. Moreover, many algal genomes and transcriptomes encode multiple POR enzymes. Phylogenetic analysis suggests that POR gene duplications occurred early in the establishment of the dinoflagellate and chlorarachniophyte lineages. Furthermore, stramenopiles and haptophytes share POR gene duplicates obtained via horizontal gene transfer from a chlorophytic alga. Phylogenetic evidence indicates that haptophytes may have obtained these genes from the stramenopiles, either by horizontal or endosymbiotic gene transfer. To elucidate the contribution of dual POR enzymes to chlorophyll synthesis, the regulation of both POR proteins (POR1 and POR2) and their mRNAs (por1 and por2) was explored for the stramenopile alga Phaeodactylum tricornutum. This alga was exposed to daily light:dark cycles at various irradiances, continuous illumination, and a light stepdown. Por1 mRNA and POR1 protein abundances were highly responsive to changes in photoperiodicity and light intensity. Transition to continuous light altered por1 mRNA regulation and suppressed POR1 protein abundance. A significantly higher abundance of POR1 protein was observed following a transition from high to low light, when algae typically photoacclimate by increasing cellular chlorophyll levels. In contrast, por2 mRNA and POR2 protein levels maintained a marked diurnal rhythmicity despite transfer to continuous light and a shift from high to low light. The sensitivity of por1/POR1 to fluctuating lighting conditions and perpetual diurnal rhythmicity por2/POR2 suggest that diatom POR1 plays a role in photoacclimation and diatom POR2 sustains daily chlorophyll synthesis. Collectively, these data evidence por gene expansions in many algal taxa and provide first insights into the por regulatory scheme of a diatom utilizing two por genes to optimize chlorophyll synthesis in response to both daily photoperiod and fluctuating environmental parameters.