A Synechococcus PCC7942 ΔccmM (Cyanophyceae) mutant pseudoreverts to air growth without regaining carboxysomes

In the cyanobacteria, intracellular structures called carboxysomes function to concentrate CO₂ around the relatively inefficient CO ₂-fixing enzyme RUBISCO. Mutants devoid of carboxysomes, such as the Synechococcus PCC7942 ΔccmM mutant, are able to grow at high-CO ₂ (2% v/v CO₂ in air) but perish in air. By growing ΔccmM in air containing 0.23% CO₂, and then normal air (0.037%) it was possible to isolate spontaneous pseudorevertant colonies (PsrΔccmM) that still lack ccmM and carboxysomes but can grow in air with a maximal doubling time ∼4-fold greater than wild type. Inorganic carbon (Ci) uptake and accumulation in PsrΔccmM was > 3-fold greater than wild type and ΔccmM. 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 PsrΔccmM 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 PsrΔccmM that might impart the growth-in-air phenotype. Moreover, the phenotype could not be conferred to ΔccmM by complementation studies with PsrΔccmM genomic DNA fragments, suggesting that it probably results from two or more, as yet unidentified, mutations. The generation of PsrΔccmM demonstrates, for the first time, that carboxysomes are not obligatory for the growth of cyanobacteria in air. We speculate that PsrΔccmM has gained some form of post-translational up-regulation of Ci transport.