Isolation and Characterization of Temperature-Sensitive, High-CO2 Requiring Mutant of Anacystis nidulans R2

Temperature-sensitive (ts), high-CO₂ requiring mutants of Anacystisnidulans R2 were isolated by N-methyl-N'-nitro-N-nitrosoguanidine(MNNG) mutagenesis and ampicillin enrichment. One of these mutantswas able to grow under ordinary air enriched with 5% CO₂, butnot under ordinary air at 40?C. At 40?C, the concentration ofCO₂ at which the rate of oxygen evolution reached half the maximumvelocity (apparent Km(CO₂) in photosynthesis) was 1,000 timeshigher in mutant cells than in wild type cells, whereas therewas no significant difference in the maximum rate of photosynthesis. When wild type and mutant cells were incubated with 55 µmNaHC0₃ under illumination at 40?C, the initial rate of inorganiccarbon (IC) transport from the medium into the cells and themaximum internal IC accumulation were significantly higher inwild type cells than in mutant cells. These results indicate that the isolated high-CO₂ requiringmutant lacks the ability in transporting IC into the cells at40?C. Furthermore, the finding that the mutant cells which aredefective in IC transport cannot grow under ordinary air suggeststhe importance of IC concentrating system(s) in photosynthesisof cyanobacteria. Present address: Department of Molecular Biology, The Universityof Wyoming, Laramie, Wyoming 82071, U.S.A. To whom reprints should be requested. (Received April 28, 1988; Accepted September 14, 1988)