Eukaryotic DNA methylation as an evolutionary device

DNA methylation is catalyzed by a family of conserved DNA methyltransferase s and is widespread among protists, plants, fungi and animals. It is howeve r absent in some species and its genomic distribution varies among organism s, Sequence comparisons suggest that known and putative eukaryotic DNA meth yltransferases fall into at least five structurally distinct subfamilies. F urthermore, it is now clear that DNA methylation can be involved in several functions, some of which may coexist within the same organism. It can inhi bit transcription initiation, arrest transcript elongation, act as an impri nting signal, and suppress homologous recombination. On the basis of these observations, we argue that DNA methylation has been conserved during evolu tion because it provides unique possibilities for setting up functions of v arious types. (C) 1999 John Wiley & Sons, Inc.