ABSTRACT
To account for the many different modified (deoxy)ribonucleosides identified so far in dif ferent types of nucleic acids, a correspondingly large number of different enzymes with distinct specificities must exist. Already 130 RNA-modification enzymes are catalogued in MODOMICS (end 2008). They correspond to more than one hundred distinct types of chemical reactions, most of which are S-AdoMet-dependent methylations of a base or a base already modified, or the 2’-hydroxyl of ribose (see Appendix 1 by Rother et al). In the case of DNA-modification enzymes, due to their considerable interests (and commercial values) in relation to restriction/modification process, the few DNA-methyltransferases from many different organisms have been characterized, purified and studied (see Appendix 1 by Rother et al). The number of identified RNA or DNA modification enzymes is increasing very fast, and within the next decade we might reasonably expect it to double or triple. How many different DNA/RNA modification enzymes exist in a given cell is still difficult to estimate, and of course, how many such enzymes exist in all types of living organisms is impossible to predict.
