Cloning of the cotA gene of Synechococcus PCC7942 and complementation of a cotA-less mutant of Synechocystis PCC6803 with chimeric genes of the two strains

cotA, a homologue of cemA that encodes a chloroplast envelope membrane protein, was cloned from Synechococcus PCC7942. The gene encodes a protein of 421 amino acids, which is similar in size to CotA of Synechocystis PCC6803 and CemA of liverwort and Chlamydomonas. There was significant sequence homology among these CotA and CemA in the C-terminal region but the homology was low in the N-terminal region. Sequencing of Synechococcus DNA in the cotA region revealed two other genes downstream of cotA, one of which is homologous to cobP and could be cotranscribed with cotA. A mutant (M48) was constructed by inactivating cotA in the wild-type (WT) Synechococcus. The mutant showed the same characteristics as the cotA-deletion mutant of Synechocystis (M29) and was unable to grow in a low sodium medium or at acidic pH under aeration with 3% CO₂in air (v/v). Synechococcus cotA did not comple-ment M29. Three chimeric cotA genes of the two cyanobacterial strains were constructed. One of these chimeric genes strongly and the other two weakly complemented the mutant.