DNA Methylation in Perspective—A Summing Up

Abstract

In this book we have presented basic information about DNA methylation. Much of this, especially with respect to eukaryotes, is intricate and complicated and unfortunately does not allow firm conclusions to be drawn as to the function of DNA methylation. Indeed, it could be that there may not simply be one function for DNA methylation, but several. On the other hand, it may not be necessary to propose that all methylated bases have a function. After all there are quite long stretches of apparently redundant DNA in eukaryotic cells, and it may very well be that a DNA methylase is actually required to act only on a limited number of bases, but by chance also methylates several others in a similar environment. In bacteria, restriction-modification involves specific base methylation, yet these account for only a minority of the methylated bases actually present in the bacterial DNA. In higher eukaryotes there is as yet no evidence for any restriction-modification system involving base methylation. Indeed it is arguable that methylation is unimportant, because some insects and fungi have extremely low levels of methylated bases. This of course overlooks the possibility that it is just this low level that is required to fulfill some vital role. In this connection it is notable that substantial methylation occurs in Neurospora crassa when transforming DNA becomes stably amplified (Bull & Wootton, 1984). It will be important in this system to establish how the N. crassa DNA methylase discriminates between the different fractions of DNA involved. This of course is also a key problem in organisms that show distinct compartments of methylated and unmethylated DNA.