A Synechococcus Pcc7942 Δ CCMM (Cyanophyceae) Mutant Pseudoreverts to AIr Growth Without Regaining Carboxysomes ¹

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