MOLECULAR ANALYSIS OF HIGH CO2 REQUIRING MUTANTS - INVOLVEMENT OF GENES IN THE REGION OF RBC, INCLUDING RBCS, IN THE ABILITY OF CYANOBACTERIA TO GROW UNDER LOW CO2

Modifications of the genomic region near (and including) rbc, the operon that codes for the large and small subunits of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), resulted in cyanobacterial mutants that demand high CO2 for growth. Mutant EK6 originated from the fusion of the 3' end of rbcS (which codes for the small subunit of Rubisco) with 84 nucleotides from the 5' flanking region of nptII (kanamycin-resistance gene), leading to a 17-kDa small subunit, as compared to 14 kDa in the wild type. Mutant D4 originated from substitution of the 1.4-kb PstI fragment, downstream of rbc, with nptII, inactivating several open reading frames in this region. Mutant O105 was obtained by chemical mutagenesis and the mutation was mapped approximately 9 kb upstream of rbc. Mutants EK6 and O105 exhibited a very low apparent photosynthetic affinity for inorganic carbon, whereas D4 had an affinity similar to that observed in wild-type cells grown under high CO2. These mutants, and the constructs used to raise them, can be used to study the role of the small subunit of Rubisco and the genomic region near rbc in cyanobacterial photosynthesis. We propose that this region contains a cluster of genes involved in the ability of cyanobacteria to grow under low ambient CO2.