Regulation of expression of the mitochondrial and peroxisomal forms of citrate synthase in maize during germination and in response to light.

Expression of genes encoding the mitochondrial and peroxisomal forms of citrate synthase (EC 2.3.3.1) was studied in maize (Zea mays L.) in scutella during germination and in leaves depending on light regime. During germination, citrate synthase activity increased in scutella both in mitochondria and in fatty-acid metabolizing peroxisomes (glyoxysomes) by day 6 and then declined. This was preceded by the peak of expression of the genes encoding the mitochondrial (Csy1) and peroxisomal (Csy2) forms of citrate synthase occurring on the day 3 of germination, after which the expression of Csy1 gradually and of Csy2 sharply declined. The decrease of expression of both genes was followed by the increase of promoter methylation which was more intensive for the gene encoding the mitochondrial form. In leaves, the activity of the mitochondrial form was much higher than that of the peroxisomal form and increased in darkness, while the peroxisomal form was almost undetectable in darkness and increased in the light. The mitochondrial form was inhibited by white and red light while the peroxisomal form was induced by white, red and blue light indicating the involvement of phytochrome and cryptochrome. The mechanism of light regulation of citrate synthase involved promoter methylation leading to the inhibition of corresponding genes and exhibiting opposite patterns for Csy1 and Csy2. Citrate synthase was purified from mitochondria and glyoxysomes of maize scutellum. The mitochondrial form had higher optimum pH as compared to the glyoxysomal form and possessed higher affinity to oxaloacetate and acetyl-CoA. It is concluded that expression of citrate synthase during germination and in response to light is regulated by methylation of promoters of corresponding genes.