Sequencing and Modification of the Gene Encoding the 42-Kilodalton Protein in the Cytoplasmic Membrane of Synechococcus PCC 7942

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Abstract
A 42-kilodalton cytoplasmic membrane protein is synthesized when high CO2-grown cells of Synechococcus PCC 7942 (Anacystis nidulans R2) are exposed to low CO2. The structural gene for this protein (cmpA) has been cloned and sequenced and shown to encode a 450 amino acid polypeptide with a molecular mass of 49 kilodalton. A deletion mutant lacking the 42-kilodalton protein was obtained by transformation of Synechococcus PCC 7942 following in vitro mutagenesis of the cloned gene. There were no significant differences between the mutant and wild-type cells in their growth rates under either low or high CO2 conditions. The activity of inorganic carbon (Ci) transport in the mutant was as high as that in the wild-type strain. In both types of cells, CO2 was the main species of Ci transported and the activities of CO2 and HCO3− transport increased when high CO2-grown cells were exposed to low CO2. We conclude that the 42-kilodalton protein is not directly involved in the Ci-accumulating mechanism of Synechococcus PCC 7942.