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ISOLATION OF ccmKLMN GENES FROM THE MARINE CYANOBACTERIUM, SYNECHOCOCCUS SP. PCC7002 (CYANOPHYCEAE), AND EVIDENCE THAT CcmM IS ESSENTIAL FOR CARBOXYSOME ASSEMBLY
Author(s) -
Ludwig Martha,
Sültemeyer Dieter,
Price G. Dean
Publication year - 2000
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1046/j.1529-8817.2000.00028.x
Subject(s) - synechococcus , biology , mutant , rubisco , synechocystis , cyanobacteria , operon , biochemistry , mutagenesis , gene , photosynthesis , genetics , bacteria
A high CO 2 requiring mutant of the marine cyanobacterium Synechococcus PCC7002 was generated using a random gene‐tagging procedure. This mutant demonstrated a reduced photosynthetic affinity for inorganic carbon (C i ) and accumulated high internal levels of C i that could not be used for photosynthesis. Analysis of the mutant genomic DNA showed that the mutagenesis had disrupted a cluster of genes involved in the cyanobacterial CO 2 concentrating mechanism (CCM), the so‐called ccm genes. These characteristics are consistent with a cyanobacterial mutant with defects in carboxysome assembly and/or functioning. Further genomic analyses indicated that the genes of the Synechococcus PCC7002 operon, ccmKLMN , are structurally similar to those of two closely related cyanobacteria, Synechococcus PCC7942 and Synechocystis PCC6803. The Synechococcus PCC7002 ccmM gene, which encodes a polypeptide with a predicted size of 70 kDa, was the direct target of the mutagenesis event. The CcmM protein has two distinct regions: an N‐terminal region that shows similarity to an archaeon gamma carbonic anhydrase and a C‐terminal region that contains repeated domains demonstrating sequence similarity to the small subunit of Rubisco. Physiological analysis of a ccmM ‐defined mutant showed that these cells were essentially identical to the original mutant; they required high CO 2 concentrations for growth, they had a low photosynthetic affinity for C i , and they internalized C i to high levels. Moreover, ultrastructural examination showed that both the original and the defined mutants lack carboxysomes. Thus, our results demonstrate that the ccmM gene of Synechococcus PCC7002 encodes a polypeptide that is essential for carboxysome assembly and therefore for proper functioning of the cyanobacterial CCM.