Properties of the promoter region of the T18^d (T13^c)Tla gene

The T18^d (formerly T13^c) gene of BALB/c mice belongs to the category of Tla genes which is expressed by both thymocytes and TL⁺ T-cell leukemias. To elucidate the regulation of T18^d, different restriction fragments of the 5' flanking region between -457 and⁺ 146 were linked to the chloramphenicol acetyltransferase (CAT) gene and transfected into TL⁺ and TL- cells. By comparison of transiently expresssed CAT activity among cells transfected with different CAT constructs, the results suggest that determination of TL⁺ vs TL phenotypes is located within the region -105 to - 33, and that an element essential to T18 d expression resides within the region - 33 to + 54. Putative DNA-binding factors characterizing particular cell types and displaying selective affinity for particular TI 8^d restriction fragments were identified by electrophoretic mobility shift assays with nuclear extracts (NEs). Two factors (or complexes) which bound specifically to the T18^d fragment-105 to - 33 were expressed preferentially in TL⁺ cells and thus may be involved in determining the tissue-selective expression of TI8^d. The close proximity of negative and positive cis-acting elements within the promoter region is consistent with regulation of T18^d gene expression by a variety of trans-acting factors whose production is attuned to development and differentiation. The data provided may serve as a guide to study the regulation of other categories of Tla genes that are normally silent in thymocytes but may become expressed by leukemia cells.