Telomerase reverse transcribes telomere repeats onto chromosome termini. The longterm goal of this project is to develop the nematode C. elegans as a system for the study of telomerase and telomere biology. C. elegans possesses holocentric chromosomes, which will allow us to determine how the initial end-to-end chromosome fusion events that occur when telomerase is deficient are created. Genetic screens have identified four proteins that are required for telomerase to function at telomeres in yeast, whereas our forward genetic screens indicate that 10 or more proteins are required in C. elegans and perhaps in other multicellular organisms. One model in the field suggests that telomeres may be sensed as DNA double-strand breaks prior to recruitment of telomerase. However, we have determined that the MRT-1 nuclease, which facilitates DNA interstrand crosslink repair, is required for telomerase to act at C. elegans telomeres. Thus, telomerase may be recruited to telomeres via a pathway that can respond to DNA interstrand crosslinks. We suspect that this hypothesis may be relevant to telomere evolution and plan to test this possibility genetically. In addition, we shall identify and characterize four new genes that are required for telomerase activity in vivo in C. elegans (279, 222g, 36f and 3211e). Purification and in vitro analysis of telomerase from C. elegans may provide functional insight regarding the biochemical consequences of the genetic defects of these telomere replication mutants. Finally, further mutant screens will be performed in an effort to saturate for genes required for telomerase activity in vivo. Telomerase only acts at telomeres for a fleeting moment during the cell cycle, so the powerful genetics of C. elegans may help to identify the suite of genes responsible for telomerase activity in vivo in multicellular organisms.