Telomeres are the special DNA-protein complexes which cap the ends of linear eukaryotic chromosomes. The telomeric DNA consists of tandem repeats with one G-rich strand and one C-rich strand. Studies from yeast and ciliates have shown that there is a mechanistic link between synthesis of the G-strand by telomerase and synthesis of the C-strand by DNA polα-primase, which results in coordinated regulation of G- and C-strand synthesis. Until recently the mechanism by which this coordinate regulation is achieved was unclear. However, insight into this aspect of telomere synthesis has come to light in the last six months both from studies with the budding yeast, S. cerevisiae and from my work with the ciliate Euplotes . I have investigated the mechanism of new telomere synthesis in Euplotes by studying the role of the lagging strand replication machinery in C-strand synthesis, and how it contributes to telomere length regulation. In Euplotes, extensive new telomere synthesis occurs during macronuclear development. This is a highly efficient, developmentally regulated process, where an entire telomere is added onto the end of a fragmented chromosome. Prior to new telomere synthesis, telomerase undergoes a programmed developmental switch and assembles into three higher order complexes with the properties required for telomere addition. The large size of the telomerase complexes together with the requirement for DNA polymerase in telomere synthesis, suggested that polα-primase might be a component of the telomerase holoenzyme. I have now shown that in Euplotes there is a direct biochemical interaction between telomerase and the lagging strand synthesis machinery. This interaction is developmentally regulated as it occurs only during the sexual stage of the Euplotes life cycle. My results show that primase is present in all three higher order telomerase complexes but absent from the vegetative telomerase complex. I also show that PCNA is present in the largest of the higher order telomerase complexes. The association of telomerase and lagging strand replication machinery within a complex explains the coordination of G- and C-strand synthesis during new telomere addition in Euplotes
