Identification of HTLV-I 21 bp repeat-specific DNA-protein complexes.

The human T cell lymphotropic virus type 1 (HTLV-I)-encoded protein, Tax, is capable of transcriptionally trans-activating HTLV-I by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21 bp sequence. Evidence suggests that Tax is incapable of directly interacting with DNA; therefore, Tax most likely regulates transcription via interaction with cellular factors. In addition to a role in Tax-mediated trans-activation, cellular factors are also critical elements in basal HTLV-I LTR-directed transcription. Therefore, characterization of cellular factors which interact with the 21 bp repeat elements is essential to understanding the molecular mechanisms involved in both basal and Tax-mediated transcription from the HTLV-I LTR. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected 21 bp repeat-specific DNA-protein complexes when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9) and monocytoid (U937) origin were reacted with each of the three 21 bp repeat elements. Furthermore, results from EMS competition analyses utilizing unlabeled 21 bp repeats as competitor DNAs have indicated a difference in the ability of each unlabeled 21 bp repeat to compete for the specific DNA-protein complexes formed between the nuclear extracts and radiolabeled 21 bp repeats. In each case, the most effective competitor was the homologous, unlabeled 21 bp repeat element. These results demonstrate that there are 21 bp repeat-specific DNA-protein complexes and suggest functional differences among the three 21 bp repeat elements.