Inactivation of the monocistronic rca gene in Anabaena variabilis suggestsa physiological ribulose bisphosphate carboxylase oxygenase activase-like function in heterocystous cyanobacteria

There was no discernible effect after incubating recombinant Anabaena Rubis co and carboxyarabinitol 1-phosphate with the product of the Anabaena rca g ene. Since the unactivated cyanobacterial Rubisco is not readily inhibited by ribulose 1,5-bisphosphate and fallover is not observed, a genetic basis for the function of the Rubisco activase-like gene (rca) was sought. The mo nocistronic rca gene was inactivated in vivo and resulting mutant strains o f A. variabilis were found to be incapable of synthesizing immunologically detected RCA protein. The requirement for the product of the rca gene in th e light was further examined by measuring Rubisco activity in permeabilized whole cells of wild-type and rca mutant strains at different light intensi ties. In a 1% CO2-air atmosphere, inactivation of rca reduced the ability o f A. variabilis to elevate Rubisco activity under high light (73 mu mol qua nta m(-2) s(-1)), but had little effect under low light (8 mu mol m(-2) s(- 1)). For air-grown cultures, differences in the rates exhibited by the wild -type and rca mutant to fully activate Rubisco during a whole-cell assay we re enhanced by increases in light intensity. The significance of the rca mu tation was underlined by effects on growth as, unlike the wild-type, growth rates did not increase after cells transferred from low to high light inte nsities. Higher exogenous CO2 concentrations (1%) were required to sustain a normal growth rate for the A. variabilis rca mutant. When grown in air le vels of CO2, the rca mutant not only needed longer times to double in cell density but also exhibited greatly diminished Rubisco activity compared wit h the wild-type strain. Despite the unusual properties of cyanobacterial Ru bisco, these results suggest a physiological role for the product of the rc a gene in maximizing the activity of Rubisco in heterocystous cyanobacteria .