EVOLUTIONARY CONSERVATION OF SEQUENCES DIRECTING CHROMOSOME BREAKAGE AND RDNA PALINDROME FORMATION IN TETRAHYMENINE CILIATES

Extensive, programmed chromosome breakage occurs during formation of t he somatic macronucleus of ciliated protozoa. The cis-acting signal di recting breakage has been most rigorously defined in Tetrahymena therm ophila, where it consists of a 15-bp DNA sequence known as Cbs, for ch romosome breakage sequence. We have identified sequences identical or nearly identical to the T. thermophila Cbs at sites of breakage flanki ng the germline micronuclear rDNA locus of six additional species of T etrahymena as well as members of two related genera. Other general fea tures of the breakage site are also conserved, but surprisingly, the o rientation and number of copies of Cbs are not always conserved, sugge sting the occurrence of germline rearrangement events over evolutionar y time. At one end of the T. thermophila micronuclear rDNA locus, a pa ir of short inverted repeats adjacent to Cbs directs the formation of a giant palindromic molecule. We have examined the corresponding seque nces from two other Tetrahymena species. We find the sequence to be pa rtially conserved, as previously implied from analysis of macronuclear rDNA, but of variable length and organization.