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Telomere proteins protect the chromosomal terminus from nucleolytic degradation and end-to-end fusion, and they may contribute to telomere length control and the regulation of telomerase. The current studies investigate the effect of Oxytricha single-stranded telomere DNA-binding protein subunits alpha and beta on telomerase elongation of telomeric DNA. A native agarose gel system was used to evaluate telomere DNA-binding protein complex composition, and the ability of telomerase to use these complexes as substrates was characterized. Efficient elongation occurred in the presence of the alpha subunit. Moreover, the alpha-DNA cross-linked complex was a substrate for telomerase. At higher alpha concentrations, two alpha subunits bound to the 16-nucleotide single-stranded DNA substrate and rendered it inaccessible to telomerase. The formation of this alpha . DNA . alpha complex may contribute to regulation of telomere length. The alpha . beta . DNA ternary complex was not a substrate for telomerase. Even when telomerase was prebound to telomeric DNA, the addition of alpha and beta inhibited elongation, suggesting that these telomere protein subunits have a greater affinity for the DNA and are able to displace telomerase. In addition, the ternary complex was not a substrate for terminal deoxynucleotidyltransferase. We conclude that the telomere protein inhibits telomerase by rendering the telomeric DNA inaccessible, thereby helping to maintain telomere length.