Abstract
A clone that transforms the RKa mutant of Synechocystis PCC6803 defective in inorganic carbon (Ci) transport to the wild-type phenotype was isolated from a cyanobacterial genomic library. The clone contained an 11.8-kilobase-pair DNA insert. Sequencing of the insert DNA in the region of the mutation in RKa revealed an open reading frame (designated as ndhB), which showed extensive amino acid sequence homology to the subunit-2 genes of NADH dehydrogenase (EC 1.6.99.3) (ndhB) of chloroplasts and mitochondria. The homology was much stronger with the chloroplast genes. Sequence analysis of the ndhB gene of RKa mutant revealed a G----A substitution that results in a Gly----Asp substitution in the deduced amino acid. A defined mutant (M55), constructed by inactivating the ndhB gene in wild-type Synechocystis, required high CO2 conditions for growth and was unable to transport CO2 and HCO3- into the intracellular Ci pool. The results indicate that the ndhB gene is required for Ci transport. Dark respiration was also depressed by the inactivation of the ndhB gene. A possible role of the ndhB gene product in the energization of Ci transport is discussed.
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