IDENTIFICATION OF A NOVEL REPRESSIVE ELEMENT IN THE PROXIMAL LCK PROMOTER

The T-cell-specific protooncogene Ick, a src-related tyrosine kinase, is under the control of two promoters that give rise to transcripts di ffering only in their 5'-untranslated regions, The distal promoter is transcriptionally active in both peripheral and thymic T-cells, wherea s expression of the proximal promoter is highest in developing thymocy tes. The proximal promoter has also been shown to be selectively activ ated in a number of colon carcinoma cell lines. Approximately 570 base pairs of proximal promoter sequence is required for expression in bot h T-cells and colon carcinoma cell lines. Protein binding studies were initiated with an oligonucleotide homologous to a region that, when d eleted, causes an increase in promoter activity in transgenic animals, Two proteins with approximate molecular masses of 35 and 75 kDa were found to bind to this region as determined by UV cross-linking studies , Absence of specific protein binding is correlated with a high level of proximal promoter expression. Competitive gel retardation analysis identified a 9-base pair binding site within the proximal Ich promoter that is necessary for repression of transcription in cells that conta in specific binding activity, Mutants of this binding site do not repr ess transcription. Repression does not occur in a cell line that expre sses Ich and lacks this activity. These data support the hypothesis th at activation of Ich transcription in colon carcinoma is due, at least in part, to the loss of a transcriptional repressor.