ADENINE METHYLATION AT DAM SITES INCREASES TRANSIENT GENE-EXPRESSION IN PLANT-CELLS

Escherichia coli encodes two major DNA methylation systems: dam, which produces 6-methyladenine; and dcm, which produces 5-methylcytosine. About 1-2% of adenine and cytosine residues in plasmid DNAs prepared in E. coli are methylated by these systems. Since DNA methylation profoundly influences gene expression in eukaryotes, we were interested in determining whether these bacterially encoded modifications might also effect plant gene expression in experimental systems. We therefore examined the influence of dam and dcm methylation on gene expression from four GUS fusion constructs in transient assays in protoplasts and microprojectile-bombarded whole tissues. In these constructs, GUS expression was driven by promoter regions derived from the Arabidopsis alcohol dehydrogenase (Adh1), maize ubiquitin (Ubi1), rice actin (Act1) and CaMV 35S genes. We show that methyladenine produced by dam methylation increased gene expression from constructs based on the Adh1, Ubi1 and Act1 genes. The increase in gene expression ranged from three-fold for Ubi1 and Adh1 in protoplasts to 50-fold for Act1 in bombarded wheat tissues. Expression of a 35S.GUS construct was, however, insensitive to dam methylation. dcm methylation had little if any effect on transient gene expression for any of these constructs. We provide indirect evidence that the critical sites of adenine methylation lie within sequences from the promoter regions, suggesting that dam methylation increases transcription rate. These results have important experimental implications and also raise the intriguing possibility that methyladenine might play a role in the regulation of gene expression in vivo.