CONSERVED DNA-SEQUENCES ADJACENT TO CHROMOSOME FRAGMENTATION AND TELOMERE ADDITION SITES IN EUPLOTES-CRASSUS

During the formation of a new macronucleus in the ciliate Euplotes cra ssus, micronuclear chromosomes are reproducibly broken at similar to 1 0 000 sites. This chromosome fragmentation process is tightly coupled with de novo telomere synthesis by the telomerase ribonucleoprotein co mplex, generating short linear macronuclear DNA molecules. In this stu dy, the sequences of 58 macronuclear DNA termini and eight regions of the micronuclear genome containing chromosome fragmentation/telomere a ddition sites were determined. Through a statistically based analysis of these data, along with previously published sequences, we have defi ned a 10 bp conserved sequence element (E-Cbs, 5'-HATTGAAaHH-3', H = A , C or T) near chromosome fragmentation sites. The E-Cbs typically res ides within the DNA destined to form a macronuclear DNA molecule, but can also reside within flanking micronuclear DNA that is eliminated du ring macronuclear development. The location of the E-Cbs in macronucle ar-destined versus flanking micronuclear DNA leads us to propose a mod el of chromosome fragmentation that involves a 6 bp staggered cut in t he chromosome, The identification of adjacent macronuclear-destined se quences that overlap by 6 bp provides support for the model. Finally, our data provide evidence that telomerase is able to differentiate bet ween newly generated ends that contain partial telomeric repeats and t hose that do not in vivo.