Catalyzes the synthesis of cardiolipin (CL) (diphosphatidylglycerol) from phosphatidylglycerol (PG) and phosphatidylethanolamine (PE). 1,2-diacyl-sn-glycero-3-phospho-(1'-sn-glycerol) + a 1,2-diacyl-sn-glycero-3-phosphoethanolamine = a cardiolipin + ethanolamine Full activity requires coexpression with the neighboring gene ymdB. Triple deletion of clsA, clsB and clsC results in a complete lack of cardiolipin, regardless of growth phase or growth conditions. All three cardiolipin synthases (ClsA, ClsB and ClsC) contribute to CL synthesis in stationary phase. Only ClsA contributes to synthesis during logarithmic growth phase. Belongs to the phospholipase D family. Cardiolipin synthase subfamily. ClsC sub-subfamily.