A Synechococcus PCC7942 Delta CCMM (Cyanophyceae) Mutant Pseudorevertsto Air Growth Without Regaining Carboxysomes

摘要

In the cyanobacteria, intracellular structures called carboxysomes function to concentrate CO sub(2) around the relatively inefficient CO sub(2)-fixing enzyme RUBISCO. Mutants devoid of carboxysomes, such as the Synechococcus PCC7942 Delta ccmM mutant, are able to grow at high-CO sub(2) (2% v-v CO sub(2) in air) but perish in air. By growing Delta ccmM in air containing 0.23% CO sub(2), and then normal air (0.037%) it was possible to isolate spontaneous pseudorevertant colonies (Psr Delta ccmM) that still lack ccmM and carboxysomes but can grow in air with a maximal doubling time similar to 4-fold greater than wild type. Inorganic carbon (Ci) uptake and accumulation in Psr Delta ccmM was >3-fold greater than wild type and Delta ccmM. However, little, or no, change was detected in the transcript abundance of known genes involved in Ci uptake. Significant differences only in the icfA and rbcLS mRNA levels were observed in Psr Delta ccmM that corresponded with measured changes in IcfA (carboxysomal carbonic anhydrase) and RUBISCO contents. Genomic sequencing spanning icfA, rbcLS, and various Ci transporter and associated gene regions did not identify mutations unique to Psr Delta ccmM that might impart the growth-in-air phenotype. Moreover, the phenotype could not be conferred to Delta ccmM by complementation studies with Psr Delta ccmM genomic DNA fragments, suggesting that it probably results from two or more, as yet unidentified, mutations. The generation of Psr Delta ccmM demonstrates, for the first time, that carboxysomes are not obligatory for the growth of cyanobacteria in air. We speculate that Psr Delta ccmM has gained some form of post-translational up-regulation of Ci transport.