TRANSCRIPTION OF THE LYMPHOCYTE-SPECIFIC TERMINAL DEOXYNUCLEOTIDYLTRANSFERASE GENE REQUIRES A SPECIFIC CORE PROMOTER STRUCTURE

The terminal deoxynucleotidyltransferase (TdT) gene encodes a template -independent DNA polymerase that is expressed exclusively in immature lymphocytes. The TdT promoter lacks a TATA box, but an initiator eleme nt (Inr) overlaps the transcription start site. The Inr directs basal transcription and also mediates activated transcription in conjunction with an upstream element called D'. We have begun to address the fund amental question of why the TdT promoter contains an Inr rather than a TATA box. First, we tested the possibility that the TdT promoter lack s a TATA box because the -30 region is needed for the binding of an es sential regulator, Mutations were introduced into the -30 region, and the? mutants were tested in transient transfection and in vitro transc ription assays. The mutations had only minor effects on promoter stren gth, suggesting that this first hypothesis is incorrect. Next, the eff ect of inserting a TATA box within the -30 region was tested. Although the TATA box enhanced promoter strength, appropriate regulation appea red to be maintained, as transcription in lymphocytes remained depende nt on the D' element. Finally, a promoter variant containing a TATA bo x at -30, but a mutant Inr, was tested. Surprisingly, transcription fr om this variant, both in vitro and in vivo, was dramatically reduced. These results suggest that the TdT promoter, and possibly other natura l promoters, contain an Inr element because one or more activator prot eins that interact with surrounding control elements preferentially fu nction in its presence.