The role of cytosine methylation in the control of nopaline synthase gene expression in a plant tumor.

The FT37/1 tumor line induced by Agrobacterium tumefaciens on flax epicotyls contains 22-24 copies of the T-DNA encoded nopaline synthase gene per cell. All the gene copies are methylated to some extent but the methylation is not uniform, nor does it reflect the methylation level of the flanking plant DNA. This extensive methylation correlates with an extremely low level of expression of the nopaline synthase gene. Treatment of the tumor line with the in vivo demethylating drug 5-azacytidine at a concentration of 3 X 10(-5) M, resulted in the demethylation of, on average, one copy of the nopaline synthase gene per cell. This demethylation was paralleled by an increase in the transcription of the gene and indicates that cytosine methylation is capable of suppressing the expression of plant genes in vivo.