One of the responses exhibited by cyanobacteria when they are limited for an essential nutrient is the rapid degradation of their light-harvesting complex, the phycobilisome. Phycobilisome degradation is an ordered proteolytic process, visible by a color change of the cyanobacterial cell from blue-green to yellow-green (chlorosis). The small polypeptide NblA plays a key role in degradation of phycobilisomes in Synechococcus sp. PCC7942. Unlike Synechococcus, Synechocystis sp. PCC6803 has two nblA-homologous genes, nblA1 and nblA2, which are contiguous on the genome. Here we show that nblA1 and nblA2 are simultaneously expressed in Synechocystis 6803 upon nitrogen deprivation, and are both required for phycobilisome degradation.
The Ca"-dependent protease of the cyanobacterium Anabuena vuriabilis is a cytoplasmic enzyme with a substrate specificity like trypsin. Its previously published DNA sequence [Maldener, I., Lockau,Genet. 225, 113-1201 contained a sequencing error. Here we report the corrected sequence which shows, that the Ca2+-protease belongs to the family of subtilases (subtilisin-like serine proteases). Consistent with its cytoplasmic localization, a pre-sequence is not found.The enzyme is produced as a precursor with a large amino-terminal propeptide. Expression of the pro-region and mature region (protease domain) in Escherichia coli cells in trans demonstrates that formation of the active enzyme requires the propeptide. The results demonstrate that propeptide-assisted protein folding also occurs with cytoplasmic enzymes, in support of the hypothesis that this mechanism is a widespread phenomenon.
The nblA family of genes encodes for small proteins necessary for the ordered degradation of phycobilisomes under certain stress conditions, a process known as chlorosis. Genes homologous to nblA seem to occur in all phycobilisome‐containing organisms. However, to date, no molecular mechanism is known for the action of NblA, nor have the gene products been characterized to understand the physical properties of the molecule and thus help elucidate the mechanism on a structural basis.
In this study we report on the first characterization of an NblA‐homologous gene product. The chromosomal gene from the cyanobacterium Anabaena sp. PCC 7120 was cloned, heterologously expressed in Escherichia coli and purified to apparent homogeneity. This allowed the protein to be characterized by analytical ultracentrifugation and CD spectroscopy. These experiments show that the NblA protein has a mostly α‐helical structure, undergoing an association reaction of folded monomers to form trimers in solution. No dimers are detectable.
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