Effects of single-stranded DNA binding proteins on primer extension by telomerase

We present a biochemical analysis of the effects of three single-stranded DNA binding proteins on extension of oligonucleotide primers by the Tetrahymena telomerase. One of them, a human protein designated translin, which was shown to specifically bind the G-rich Tetrahymena and human telomeric repeats, slightly stimulated the primer extension reactions at molar ratios of translin/primer of <1:2. At higher molar ratios, it inhibited the reactions by up to 80%. The inhibition was caused by binding of translin to the primers, rather than by a direct interaction of this protein with telomerase. A second protein, the general human single-stranded DNA binding protein Replication Protein A (RPA), similarly affected the primer extension by telomerase, even though its mode of binding to DNA differs from that of translin. A third protein, the E. coli single-stranded DNA binding protein (SSB), whose binding to DNA is highly cooperative, caused more substantial stimulation and inhibition at the lower and the higher molar ratios of SSB/primer, respectively. Both telomere-specific and general single-stranded DNA binding proteins are found in living cells in telomeric complexes. Based on our data, we propose that these proteins may exert either stimulatory or inhibitory effects on intracellular telomerases, depending on their local concentrations.