Effect of CWG methylation on expression of plant genes

The presence of two DNA methyltransferases in Pisum raises the possibility that they serve different functions. In vitro methylation of CWG sequences in the strong cauliflower mosaic virus 35S promoter had no effect on report er gene expression. In contrast, in vitro methylation of CWG sequences in t he relatively weak, CG-deficient Phaseolus vulgaris rbcS2 promoter inhibite d transcription. Expression of both constructs was strongly inhibited by ex tensive CG methylation. A search of published plant promoter sequences reve aled that the CG content of promoters is very variable, with some promoters having typical CG islands. In contrast, the distribution of CWG sequences is more even with little evidence for CWG islands.