Telomeres are specialized protein-DNA structures that compose the very ends of linear eukaryotic chromosomes. Telomeres are essential for chromosome stability in that they protect the ends of chromosomes from degradation and recombination and participate in processes that restore terminal nucleotides that are lost during DNA replication. When telomere length and structure are not properly maintained, cells lose their ability to divide and become senescent. Lack of telomere maintenance in normal human cells appears to limit their proliferative potential, which has important implications for the processes of aging and cancer. We are investigating the precise three-dimensional structures of single-strand DNA-protein complexes that form the extreme 3'-termini of telomeres and, therefore, constitute the very ends of chromosomes. The ends of Oxytricha nova telomeres contain a 16 nucleotide single- strand 3'-DNA extension of the telomeric (TTTTGGGG)n DNA that is bound tightly by a two subunit protein referred to as the telomere binding protein. We have determined crystal structures of a ternary complex containing the alpha and beta subunits of the O. nova telomere binding protein complexed with telomeric GGGGTTTTGGGG DNA (the alpha-beta-DNA complex) and of the alpha subunit complexed with TTTTGGGG DNA (the alpha-DNA complex). We now propose to: (i) complete our structural work on these unique protein-ssDNA complexes (ii) investigate biochemically and structurally the many interesting protein-protein and protein- nucleic acid interactions observed in these complexes, and (iii) explore the intriguing possibility the the alpha-beta-DNA ternary complex forms via an ordered assembly pathway as suggested by biochemical experiments. These studies will yield new insights into telomere structure and function, single-strand DNA binding and recognition, and kinetic pathways for assembling complex macromolecular nucleoprotein structures.