Uncultured marine cyanophages encode for active NblA, phycobilisome proteolysis adaptor protein

Abstract

SummaryPhycobilisomes (PBS) are large water-soluble membrane-associated complexes in cyanobacteria and some chloroplasts that serve as a light-harvesting antennas for the photosynthetic apparatus. When short of nitrogen or sulfur, cyanobacteria readily degrade their phycobilisomes allowing the cell to replenish the vanishing nutrients. The key regulator in the degradation process is NblA, a small protein (~6 kDa) which recruits proteases to the PBS. It was discovered previously that not only do cyanobacteria possessnblAgenes but also that they are encoded by genomes of some freshwater cyanophages. A recent study, using assemblies from oceanic metagenomes, revealed genomes of a novel uncultured marine cyanophage lineage which contain genes coding for the PBS degradation protein. Here, we examine the functionality ofnblA-like genes from these marine cyanophages by testing them in a freshwater model cyanobacterialnblAknockout. One of the viral NblA variants could complement the non-bleaching phenotype and restore PBS degradation. Our findings reveal a functional NblA from a novel marine cyanophage lineage. Furthermore, we shed new light on the distribution ofnblAgenes in cyanobacteria and cyanophages.Originality-Significance StatementThis is the first study to examine the distribution and function ofnblAgenes of marine cyanophage origin. We describe as well the distribution ofnblA-like genes in marine cyanobacteria using bioinformatic methods.