A SINGLE TELOMERASE RNA IS SUFFICIENT FOR THE SYNTHESIS OF VARIABLE TELOMERIC DNA REPEATS IN CILIATES OF THE GENUS PARAMECIUM

Paramecium telomeric DNA consists largely of a random distribution of TTGGGG and TTTGGG repeats, Given the precise nature of other ciliate t elomerases, it has been postulated that there are two distinct types o f the Paramecium enzyme, each synthesizing perfect telomeric repeats: one with a template RNA that Specifies the addition of TTTGGG and the second dictating the synthesis of TTGGGG repeats, We have cloned and s equenced telomerase RNA genes from Paramecium tetraurelia, P. primaure lia, P. multimicronucleatum, and P. caudatum. Surprisingly, a single g ene encodes telomerase RNA in all four species, although an apparently nontranscribed pseudogene is also present in the genome of P. primaur elia, The overall lengths of the telomerase RNAs range between 202 and 209 nucleotides, and they can be folded into a conserved secondary st ructure similar to that derived for other ciliate RNAs, All Paramecium telomerase RNAs examined include a template specific for the synthesi s of TTGGGG telomeric repeats, which has not been posttranscriptionall y edited to account for the conventional synthesis of TTTGGG repeats, On the basis of these results, possible mechanisms for the synthesis o f variable telomeric repeats by Paramecium telomerase are discussed.