Gel shift and UV cross-linking analysis of Tetrahymena telomerase

Telomerase is an unusual ribonucleoprotein that synthesizes new telomeres onto chromosome ends. The enzyme has been most extensively characterized in ciliates, where the RNA component has been cloned from several species, and its elongation properties have been characterized in detail. To understand the substrate specificity and protein composition of telomerase, we have used gel shift and UV cross-linking to characterize the enzyme from the ciliate Tetrahymena thermophila. In a mobility shift assay, a complex was identified that contained telomerase RNA, co-purified with telomerase activity, and was sensitive to nuclease treatment. The mobility shift complexes specifically formed using several different single-stranded, telomeric sequences but not non-telomeric primers. These results suggest that the specificity of telomerase for G-rich primer sequences occurs at least in part at the level of primer binding. UV cross-linking analysis identified a 100-kDa cross-linked protein that may be a telomerase component.

Photoaffinity labeling of terminal deoxynucleotidyl transferase. 1. Active site directed interactions with 8-azido-2'-deoxyadenosine 5'-triphosphate

A photoaffinity analogue of dATP, 8-azido-2'-deoxyadenosine 5'-triphosphate (8-azido-dATP), was used to probe the nucleotide binding site of the non-template-directed DNA polymerase terminal deoxynucleotidyl transferase (EC 2.7.7.31). The Mg2+ form of 8-azido-dATP was shown to be an efficient enzyme substrate with a Km of 53 microM. Loss of enzyme activity occurred during UV photolysis only in the presence of 8-azido-dATP. At saturation (120 microM 8-azido-dATP), 54% of the protein molecules were modified as determined by inhibition of enzyme activity. Kinetic analysis of enzyme inhibition induced by photoincorporation of 8-azido-dATP indicated an apparent Kd of approximately 38 microM. Addition of 2 mM dATP to 120 microM 8-azido-dATP resulted in greater than 90% protection from photoinduced loss of enzyme activity. In contrast, no protection was observed with the addition of 2 mM dAMP. Enzyme inactivation was directly correlated with incorporation of radiolabeled 8-azido-dATP into the protein and UV-induced destruction of the azido group. Photoincorporation of 8-azido-dATP into terminal transferase was reduced by all purine and pyrimidine deoxynucleoside triphosphates of which dGTP was the most effective. The alpha and beta polypeptides of calf terminal transferase were specifically photolabeled by [gamma-32P]-8-azido-dATP, and both polypeptides were equally protected by all four deoxynucleoside triphosphates. This suggests that the nucleotide binding domain involves components from both polypeptides.

Biochemistry of terminal deoxynucleotidyltransferase. Identification and unity of ribo- and deoxyribonucleoside triphosphate binding site in terminal deoxynucleotidyltransferase

Terminal deoxynucleotidyltransferase is the only DNA polymerase that is strongly inhibited in the presence of ATP. We have labeled calf terminal deoxynucleotidyltransferase with [32P]ATP in order to identify its binding site in terminal deoxynucleotidyltransferase. The specificity of ATP cross-linking to terminal deoxynucleotidyltransferase is shown by the competitive inhibition of the overall cross-linking reaction by deoxynucleoside triphosphates, as well as the ATP analogs Ap4A and Ap5A. Tryptic peptide mapping of [32P]ATP-labeled enzyme revealed a peptide fraction that contained the majority of cross-linked ATP. The properties, chromatographic characteristics, amino acid composition, and sequence analysis of this peptide fraction were identical with those found associated with dTTP cross-linked terminal deoxynucleotidyl-transferase peptide (Pandey, V. N., and Modak, M. J. (1988a). J. Biol. Chem. 263, 3744-3751). The involvement of the same 2 cysteine residues in the crosslinking of both nucleotides further confirmed the unity of the ATP and dTTP binding domain that contains residues 224-237 in the primary amino acid sequence of calf terminal deoxynucleotidyltransferase.

Biochemistry of terminal deoxynucleotidyl transferase. Conditions for and characterization of ultraviolet light mediated substrate cross-linking to terminal deoxynucleotidyl transferase

Calf thymus terminal deoxynucleotidyl transferase may be cross-linked to substrate deoxynucleoside triphosphate as well as ATP in a reaction mediated by ultraviolet irradiation. The cross-linking requires the presence of divalent cation but is independent of primer. The cross-linking of substrate to enzyme is extremely sensitive to substrate binding site directed reagents, e.g. pyrophosphate and pyridoxal 5'-phosphate, and to o-phenanthroline, a zinc-directed chelator. The cross-linking reaction is competitively inhibited by all four deoxynucleoside triphosphates, ATP, and GTP, but is resistant to sulfhydryl reagents. When primer is provided in the reaction mixture, cross-linking of both substrate and primer to 26K subunit of terminal deoxynucleotidyl transferase may be demonstrated.

Terminal deoxynucleotidyl transferase activity in the regenerating thymus of x-irradiated mice

The distribution of terminal deoxynucleotidyl transferase (TdT) enzyme activity (EU per 10 8 cells) between peaks I and II was followed for a period of 42 days in regenerating thymus of lethally irradiated (9 Gy) C3H mice restored with 10 6 (C3H x AKR) F1 bone marrow cells. The detection of Thy-1.1 and Thy-1.2 surface antigens allowed for the discrimination between host and donor cells, and the main subpopulations of thymic cells were characterized by their sensitivity to H-2 k antiserum and to dexamethazone. Two peaks of TdT activity could be detected on phosphocellulose chromatographic separation. The distribution of TdT activity between these two peaks was followed during the two periods of thymic endo- and exoregeneration. Peak I TdT activity was closely correlated with the variation in the percentage of high H-2 population. Peak II activity was mostly related to low H-2 cells. The per cell content of both peak I and peak II activities exceeded the norm in rapidly expanding populations. Finally between days 10 and 14 the TdT activity of the endoregenerating population was apparently not different from that of the exoregenerating population between days 14 and 22.