DNA methylation. The effect of minor bases on DNA-protein interactions.

DNA methylation is found almost ubiquitously in nature and the methyltransferases show evidence of a common evolutionary origin. It will be a fascinating study in protein evolution to follow the ways in which the structures of the various enzymes have developed. Although methylation may have a direct effect on DNA structure the evidence for the importance of this in vivo is accumulating only slowly. In contrast, there is now abundant evidence that methylation of DNA affects DNA-protein interactions and so may have a function in all processes in which such interactions occur. The binding of nucleases is affected in the processes of mismatch repair, DNA restriction and possibly demethylation during differentiation in vertebrates. The binding of transcription factors is affected by DNA methylation and the association of DNA with packaging and segregation proteins may play a part in the control of transcription and replication. The interplay of these effects makes DNA methylation a complex but rewarding area for study. Perhaps we should no longer refer to methylcytosine and methyladenine as minor bases, but rather as key bases which help regulate the functions of DNA.