Transcriptional regulation and DNA methylation of nuclear genes for photosynthesis in nongreen plant cells.

The transcripts of nuclear genes for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS), chlorophyll a/b-binding protein (cab), and extrinsic 33-kDa protein involved in photosystem II water oxidation (woxA) were not detectable in the white wild cultured cells of sycamore (Acer pseudoplatanus), in contrast to their high levels in the sibling green mutant cells and the constitutive expression of actin genes (act) in both cell types. We have examined the template activities of nuclear DNAs using the HeLa cell in vitro transcription system. All of the three photosynthesis genes from the green cell line and act from both cell types were well transcribed in vitro, but these photosynthesis genes from the white cell line were not, indicating that the transcriptional regulation is ascribable to DNA templates. Digestion of nuclear DNA with methyl-sensitive and -insensitive isoschizomeric endonucleases and the subsequent Southern hybridization showed that each gene has the identical recognition sites of restriction enzymes in the green and white cell lines, but some of the sites were methylated only in the photosynthesis genes in the white cells. There was observed a clear inverse relationship between the level of expressed transcripts and the extent of DNA methylation. Thus, it is inferred that the selective methylation of DNA is a likely mechanism for suppressing transcription of nuclear genes for photosynthesis in the nonphotosynthetic plant cells.