METHYLATION SITES IN ANGIOSPERM GENES

The extent to which CpG dinucleotides were depleted in a large set of angiosperm genes was, on average, very similar to the extent of CpG depletion in total angiosperm genomic DNA and far less than the extent of CpG depletion in vertebrate genes. Gene sequences from Arabidopsis thaliana, a dicotyledonous species with relatively low levels of total 5-methylcytosine, were just as CpG depleted as the angiosperm genes in general. Furthermore, levels of TpG and CpA, the potential deamination mutation products of methylated CpG, were elevated in A. thaliana genes, supporting a high rate of deamination mutation as the cause of the CpG deficiency. Using a method that takes into account the dinucleotide frequencies within each sequence of interest, we calculated the expected frequencies of CpNpG trinucleotides, which are also highly methylated in angiosperm genomes. CpNpG trinucleotides were not extensively enriched or depleted in the angiosperm genes. Two hypotheses could account for our results. Differential depletion of CpG and CpNpG within angiosperm genes and differential depletion of CpG in angiosperm and vertebrate genes could arise from different efficiencies of mismatch repair or from different levels of cytosine methylation in the cell lineages that contribute to germ cells.