Evidence for tissue-specific cytosine-methylation of plastid DNA from Zea mays

Abstract

Total DNA isolated from leaves, etiolated seedlings, roots, endosperm or embryos of Zea mays was digested separately with each of the restriction enzymes HpaII, MspI and HhaI, and the resulting fragment patterns, which were specific for the plastid rRNA operon, were analyzed by Southern hybridization. While most of the fragment patterns were consistent with previously established physical maps, the partial resistance shown by one HpaII site and one HhaI site, both of which reside in the 16S/23S rDNA spacer region, was observed in DNA isolated from embryo, root tissue and endosperm. The partially resistant HpaII site was susceptible to cleavage with restriction enzyme MspI. From this and from the known inhibition of restriction enzyme Hhal at methylated HhaI sites, we conclude that the partial resistance of the two sites is caused by C-specific methylation of plastid DNA in the respective tissues. The tissue specificity of this DNA methylation is likely to reflect a differential expression of plastome encoded genes.