A clpP1 inactivation mutant in the cyanobacterium Synechococcus sp. PCC 7942 is impaired in phycobilisome degradation during sulfur-deficient growth

Abstract

When deprived of an essential nutrient cyanobacteria undergo a process known as chlorosis, in which phycobilisome degradation occurs. To date the identity of the protease(s) involved in phycobilisome degradation has remained elusive. One possible candidate is the Clp protease, which is composed of a proteolytic subunit as well as a regulatory ATPase subunit. There are three isomers of the gene for the proteolytic subunit in Synechococcus sp. PCC 7942, clpP1-3. Two Clp proteins, ClpC and ClpX, are likely to act as partners in Clp proteolytic complexes as the ATPase regulatory subunits. Here we report that Synechococcus sp. PCC 7942 cells in which the clpP1 gene has been inactivated do not degrade their phycobilisomes to the same extent as wild-type cells when starved of sulfur. In wild-type cultures the level of phycocyanin decreases by 85% within 48 hours whereas in ?clpP1 cultures phycocyanin content drops by only 20%. In addition, ClpP1 can be detected in isolated phycobilisome preparations after growth in sulfur-deplete media but not after growth in sulfur-replete media .The expected ATPase regulatory partners for the Clp protease in cyanobacteria, Clp X and ClpC, cannot be detected in isolated phycobilisomes. However, the molecular chaperone GroEL can be found in phycobilisomes isolated after growth in both sulfur replete and sulfur deplete media although the amount of GroEL increases after growth in sulfur-deplete media. We propose, therefore, that ClpP1 and GroEL co-operate in the degradation of phycobilisomes during sulfur limitation.