La. Klobutcher et al., CONSERVED DNA-SEQUENCES ADJACENT TO CHROMOSOME FRAGMENTATION AND TELOMERE ADDITION SITES IN EUPLOTES-CRASSUS, Nucleic acids research, 26(18), 1998, pp. 4230-4240
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.