Terminal deoxynucleotidyl transferase during the development of chicken thymus

T-cell specific avian TdT: characterization of the cDNA and recombinant enzyme

A cDNA clone coding for avian terminal deoxynucleotidyl transferase (TdT) has been isolated and sequenced. The size of this cDNA was 2545 bp with an open reading frame of 1521 bp and a predicted translation product of 58 kDa. Comparison of this TdT sequence with other known TdT sequences has revealed a very high degree of homology at both the DNA and predicted amino acid levels. The chicken TdT cDNA was expressed in a bacterial system and the protein was purified by affinity chromatography. The purified recombinant enzyme, with a specific activity of approximately 1700 U/mg protein, was significantly less active than TdTs from mammalian species. This finding correlates with the observation that TdT isolated from avian thymus has lower activity than that isolated from any mammalian thymus source. Northern blot hybridization analyses and reverse transcription PCR of RNA preparations were carried out with the chicken cDNA. The data generated from these experiments revealed that the TdT RNA was only expressed in the thymus and not in the bone marrow or the bursa of Fabricius during pre- and post hatching chicken development. These data suggest that while TdT is probably involved in N region addition in chicken T-cell receptor genes, it is unlikely to play a role in diversification of immunoglobulin genes.

The intracellular location of terminal transferase does not vary with cell cycle stage

The biochemical activity of terminal transferase (TdT) in the thymocytes of leukemic AKR mice has no relationship to cell cycle stage, unlike the activity of replicative DNA polymerase which increases during the period of DNA synthesis. Moreover, such assays of DNA polymerase alpha reveal a shift in enzyme activity from cytoplasm to nucleus during S phase. In the present study, the role of TdT in DNA metabolism was explored further by examining the intracellular location of the enzyme during cytokinesis. Single cell suspensions of thymocytes from leukemic AKR mice were partially synchronized by velocity sedimentation in a sucrose gradient at unit gravity and harvested according to cell cycle stage. The content and location of TdT in individual cells was determined by indirect immunofluorescence using a rabbit antiserum to calf thymus TdT as the primary antibody. There was no relationship of fluorescence intensity or of the proportion of TdT-positive cells to cell cycle stage. In all samples examined (n = 6) the enzyme was located almost entirely in the nucleus throughout cytokinesis. These results do not support the hypothesis that the intracellular location of TdT may vary with cell cycle stage and a role for the enzyme in DNA synthesis remains to be defined.

Asymmetric division of immature lymphoid cells of the embryonic mouse thymus

Cell division of thymus lymphoid cells from 15- to 19-day-old embryonic mice, as well as from 2- and 7-day-old mice was investigated on cell smears stained with Giemsa or with the Feulgen reaction followed with malachite green. Asymmetrically dividing cells were observed during the development. At the anaphase and telophase of such cells, the nucleus of one of two immature daughter cells was apparently smaller and more condensed than that of the counterpart cell. The asymmetry was already apparent at the level of sister chromosomes at anaphase. Sometimes a difference also existed in the cytoplasm: the cytoplasm of the smaller immature daughter cell with condensed nucleus was stained with Giemsa more deeply than that of the counterpart. The frequency of cells displaying apparent asymmetric division among total dividing cells at the telophase was around 10% between 15 and 19 days of gestation, and decreased to 2% on days 2 and 7 after birth. Asymmetrically dividing cells belonged to immature lymphoid cells of larger size. The biological implications of the present phenomenon are discussed.