A small polypeptide triggers complete degradation of light-harvesting phycobiliproteins in nutrient-deprived cyanobacteria

Phycobilisomes are the multiprotein complexes predominantly responsible for harvesting light energy in cyanobacteria and some eukaryotic algae. When the cyanobacterium Synechococcus sp. strain PCC 7942 is deprived of an essential nutrient, the phycobilisomes are specifically and rapidly degraded. Degradation may be either partial (after phosphorus deprivation) or complete (after sulfur or nitrogen deprivation). We have developed a visual screen to obtain mutants unable to degrade their phycobilisomes upon nutrient starvation. Complementation of one of these mutants led to the identification of a gene, designated nblA, that encodes a 59 amino acid polypeptide essential for phycobilisome degradation. Transcription of nblA increases dramatically in sulfur-or nitrogen-deprived cells and moderately in phosphorus-deprived cells. Using the phosphorus-regulated alkaline phosphatase (phoA) promoter as a tool, we engineered constructs from which we could control the expression of either sense or antisense nblA. Increased expression of sense nblA caused complete phycobilisome degradation during phosphorus deprivation, while expression of antisense nblA prevented phycobilisome degradation. Hence, nblA is necessary, and may be sufficient, for the degradation of phycobilisomes under adverse environmental conditions. Further investigation of the mechanism by which nblA causes phycobilisome destruction may reveal general principles that govern the specificity of macromolecular complex degradation.