The T18d (formerly T13c) 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 T18d, 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 8d restriction fragments were identified by electrophoretic mobility shift assays with nuclear extracts (NEs). Two factors (or complexes) which bound specifically to the T18d fragment-105 to - 33 were expressed preferentially in TL+ cells and thus may be involved in determining the tissue-selective expression of TI8d. The close proximity of negative and positive cis-acting elements within the promoter region is consistent with regulation of T18d 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.
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