Phycobilisomes (PBS) are the major light-harvesting complexes of cyanobacteria. These usually blue-coloured multiprotein assemblies are rapidly degraded when the organisms are starved for combined nitrogen. This proteolytic process causes a colour change of the cyanobacterial cells from blue-green to yellow-green (‘bleaching’). As is well documented for the unicellular, non-diazotrophic cyanobacteria Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803, a gene termed nblA plays a key role in PBS degradation. Filamentous, diazotrophic cyanobacteria like Anabaena adapt to nitrogen deprivation by differentiation of N2-fixing heterocysts. However, during the first hours after nitrogen deprivation all cells degrade their PBS. When heterocysts mature and nitrogenase becomes active, vegetative cells resynthesize their light-harvesting complexes while in heterocysts the phycobiliprotein content remains very low. Expression and function of nblA in Anabaena sp. PCC 7120 was investigated. This strain has two nblA homologous genes, one on the chromosome (nblA) and one on plasmid delta (nblA-p). Northern blot analysis indicated that only the chromosomal nblA gene is up-regulated upon nitrogen starvation. Mutants with interrupted nblA and nblA-p genes, respectively, grew on N2 and developed functional heterocysts. Mutant ΔnblA-p behaved like the wild-type. However, mutant ΔnblA was unable to degrade its PBS, which was most obvious in non-bleaching heterocysts. The results show that NblA, encoded by the chromosomal nblA gene, is required for PBS degradation in Anabaena but is not essential for heterocyst differentiation.

20S proteasomes are multifunctional proteinase complexes ubiquitous in eucaryotes. We have cloned the yeast PRE3 gene by complementation of the pre3‐2 mutation, which leads to a defect in the peptidylglutamyl‐peptide hydrolyzing activity of the 20S proteasome. The PRE3 gene, a β‐type member of the proteasomal gene family, is essential for cellular life and codes for a 193‐amino acid proteasomal subunit with a predicted molecular mass of 21.2 kDa. The Pre3 protein shows striking homology to the human proteasome subunits Hsδ and Lmp2 (Ring12). Lmp2 is encoded in the major histocompatibility complex class II region implicating proteasomes in antigen processing.

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