DNA methylation as a mechanism of transcriptional regulation in nonphotosynthetic plastids in plant cells.

Transcription of amyloplast DNA in a heterotrophic line of cultured cells of sycamore (Acer pseudoplatanus L.) appeared to be greatly suppressed. A mutant cell line obtained from the heterotrophic line is green and autotrophic. Heavy modification of amyloplast DNA with a variety of methylated bases was demonstrated by analysis of the acid hydrolysate of DNA by high-performance liquid chromatography, but little modification of chloroplast DNA from the green line was detected. When plastid DNAs from the original and green cell lines were digested with methyl-sensitive restriction enzymes, DNA methylation was detected in regions containing the genes for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL), subunits of chloroplast coupling factor 1 (atpA, -B, and -E), the apoprotein of P700 (psaA), and ribosomal protein S4 (rps4) but not the genes for 16S rRNA and the 32-kDa QB protein (psbA) in the original line, whereas no methylation was observed in the green line. The genes for which methylation was not detectable were found to be active as templates for in vitro transcription by Escherichia coli RNA polymerase, but the methylated genes were apparently inactive. Methylation of DNA is a likely mechanism for the regulation of expression of amyloplast DNA in sycamore cells.