BIOLOGY OF DNA RESTRICTION

Our understanding of the evolution of DNA restriction and modification systems, the control of the expression of the structural genes for th e enzymes, and the importance of DNA restriction in the cellular econo my has advanced by leaps and bounds in recent years. This review docum ents these advances for the three major classes of classical restricti on and modification systems, describes the discovery of a new class of restriction systems that specifically cut DNA carrying the modificati on signature of foreign cells, and deals with the mechanisms developed by phages to avoid the restriction systems of their hosts.