The NblAl protein from the filamentous cyanobacterium Tolypothrix PCC 7601: Regulation of its expression and interactions with phycobilisome components

Abstract

Cyanobacteria respond to changes in light or nutrient availability by modifications in their photosynthetic light harvesting antenna. In unicellular cyanobacteria a small polypeptide (NblA) is required for phycobilisome degradation following environmental stresses. In the filamentous strain Tolypothrix sp. PCC 7601 the nblAl gene, encoding a NblA homologue, is located upstream of the operon coding for phycoerythrin (cpeBA). The nblAl transcripts all originate from a single transcription start point; their intracellular levels vary according to nitrogen regimes but not with light spectral quality. Using recombinant His-tagged NblAl protein, we found that in vitro NblAl has affinity for both phycocyanin and phycoerythrin subunits from Tolypothrix sp. PCC 7601, but not for allophycocyanin from this cyanobacterium or for phycobiliproteins from other cyanobacterial species. We also observed that although nblAl is mainly expressed under nitrogen starvation, NblAl polypeptides are always present in the cell; a significant portion of them co-purify with phycobilisome preparations but only if cells were grown under red light. Our data indicate that NblAl attaches to the phycobilisomes even under non-inducing conditions and suggest a preferential affinity of NblAl for phycocyanin.