Abstract
Transcriptional regulation has been shownto operate as a selective control mechanismof expression of photosynthetic genes in the nonphotosynthetic plastids, amyloplasts, of a white-wild cell line of sycamore (Acer pseudoplatanus L.). To elaborate the mechanisms governing the transcriptional regulation at the molecular level, we have examined the template activity of the amyloplast DNA compared to the chloroplast DNA by using the in vitro run-off transcription assay system with extracts of the two plastid types. The results of these assays clearly indicate that most of the amyloplast DNA regions do not serve as a template for the in vitro transcription regardless of the plastid extracts; this is in contrast to the chloroplast DNA which serves as an active template. It is highly likely that the template activity of amyloplast DNA per se is the modulating element of transcriptional regulation. Parallel experiments determining the DNA base content by HPLC analysis have shown that a variety of methylated bases, especially 5-methylcytosine, are localized in the DNAregions containing suppressed genes of the amyloplast genome. In sharp contrast, methylated bases were undetectable in the expressed gene regions of amyloplast and whole chloroplast genomes. The overall findings strongly support the notion that DNAmethylation is involved in the selective suppression of photosynthetic genes in the nonphotosynthetic plastids of cultured sycamore cells.
