A mutant of Anabaena sp. strain PCC7120 requiring high
CO2 was generated using Tn5 mutagenesis. This is the
first data for a filamentous cyanobacterium. The mutant was capable of growing
at 5% CO2, but incapable of growing at air levels
of CO2. Southern hybridization analysis indicated that
the Anabaena genome was inserted by the transposon at
one site. The apparent photosynthetic affinity of the mutant to external
dissolved inorganic carbon (DIC) was about 300 times lower that of the wild
type (WT), and the medium alkalization rate as well as the carboxysomal
carbonic anhydrase activity of the mutant was also lower than those of the WT.
When the mutant was transferred from the culture medium bubbled with 5%
CO2 to higher DIC (8.4%
CO2) or 1% CO2, it showed
similar responses to the WT. However, aberrant carboxysomes were found in the
mutant cells through ultrastructural analysis, indicating it was most probably
the wrong organization of the carboxysomes that eventually led to the
inefficient operation of carboxysomal carbonic anhydrase and the subsequent
defectiveness of the mutant in utilizing DIC.
Intracellular Carbonic Anhydrase Is Essential to Photosynthesis in Chlamydomonas reinhardtii at Atmospheric Levels of CO2 (Demonstration via Genomic Complementation of the High-CO2-Requiring Mutant ca-1)