EN
Although the delta ribozymes have been stud ied for more than ten years the most im- por tant in for ma tion con cern ing their struc ture and mech a nism of ca tal y sis were only ob tained very re cently. The crys tal struc ture of the genomic delta ribozyme turns out to be an ex cel lent ex am ple of the ex traor di nary prop er ties of RNA mol e cules to fold into uniquely com pact struc tures. De tails of the X-ray struc ture have greatly stim u- lated fur ther stud ies on the fold ing of the ribozymes into func tion ally ac tive mol e cules as well as on the mech a nism of RNA ca tal y sis. The abil ity of the delta ribozymes to carry out gen eral acid-base ca tal y sis by nu cle o tide side chains has been as sumed in two pro posed mech a nisms of self-cleavage. Re cently, con sid er able prog ress has been also made in char ac ter iz ing the cat a lytic prop er ties of trans-act ing ribozyme vari ants that are po ten tially at trac tive tools in the strat egy of di rected RNA degradation.