A MUTANT ISOLATED FROM THE CYANOBACTERIUM SYNECHOCOCCUS PCC7942 IS UNABLE TO ADAPT TO LOW INORGANIC CARBON CONDITIONS

Using a novel screening procedure, we have selected a new class of mut ant from the cyanobacterium Synechococcus PCC7942 that fails to adapt to growth at an extremely low inorganic carbon (C-i) concentration. Th e mutant (Tm17) reported in this study grows normally at or above air levels of CO2 (340 mu L L(-1)) but does not survive at 20 mu L L(-1) C O2 in air. Air-grown Tm17 cells showed properties similar to wild-type cells in various aspects of the CO2- concentrating mechanism examined . Following transfer from air levels to 20 mu L L(-1) CO2, however, th e mutant cells failed to increase their photosynthetic affinity for C- i. This results in an approximately 10-fold difference in photosynthet ic affinity between the wild-type and Tm17 cells under C-i-limiting co nditions [the K-0.5(C-i) values were 11 and 136 mu M, respectively]. F urther examination of factors possibly contributing to this low photos ynthetic affinity showed that Tm17 cells have no inducible high-affini ty HCO3- transport and do not appear to show induction of increased ca rboxysomal carbonic anhydrase and ribulose-l,5-bisphosphate carboxylas e/oxygenase activities. It appears that a common factor, possibly rela ting to CO2 detection and/or induction signal, or the HCO3- transport mechanism may have been impaired in the mutant. Complementation result s indicate that the mutation responsible for the phenotype has occurre d in an 8- to 10-kb EcoRl genomic DNA fragment.