Cardiolipin synthase expression is essential for growth at elevated temperature and is regulated by factors affecting mitochondrial development

Cardiolipin (CL) is a unique dimeric phospholipid localized primarily in th e mitochondrial membrane. In eukaryotes, the enzyme CL synthase catalyses t he synthesis of CL from two lipid substrates, CDP-diacylglycerol and phosph atidylglycerol. In earlier studies, we reported the purification of CL synt hase from Sao charomyces cerevisiae and the cloning of the gene CRD1 (previ ously called CLS1) that encodes the enzyme. Because CL is an important comp onent of the mitochondrial membrane, knowledge of its regulation will provi de insight into the biogenesis of this organelle. To understand how CL synt hesis is regulated, we analysed CRD1 expression by Northern blot analysis o f RNA extracted from cells under a variety of growth conditions. CRD1 expre ssion is regulated by mitochondrial development factors. CRD1 levels were 7 - to 10-fold greater in stationary than in logarithmic growth phase, and th reefold greater in wildtype than in rho(0) mutants. Expression was somewhat elevated during growth in glycerol/ethanol versus glucose media. In contra st, CRD1 expression was not regulated by the phospholipid precursors inosit ol and choline, and was not altered in the regulatory mutants ino2, ino4 an d opi1. Mutations in cytochrome oxidase assembly, which led to reduced Crd1 p enzyme activity, did not affect CRD1 expression. The crd1 null mutant mak es a truncated CRD1 message. Although the null mutant can grow on both ferm entable and non-fermentable carbon sources at lower temperatures, it cannot form colonies at 37 degrees C. In conclusion, CRD1 expression is controlle d by factors affecting mitochondrial development, but not by the phospholip id precursors inositol and choline. Expression of CRD1 is essential for gro wth at elevated temperatures, suggesting that either CL or Crd1p is require d for;an essential cellular function.