COMPLEX REGULATION OF HUMAN C-KIT TRANSCRIPTION BY PROMOTER REPRESSORS, ACTIVATORS, AND SPECIFIC MYB ELEMENTS

The c-kit proto-oncogene is expressed in several tissues during develo pment. To understand the mechanisms controlling the expression of this gene, we characterized the human c-kit promoter. Expression is contro lled transcriptionally, The 5'-flanking DNA was used to make promoter deletion-reporter constructs that were tested in cells that were eithe r positive or negative for endogenous c-Kit. The results demonstrate t hat DNA, to at least position -4100, directs transcription well in bot h positive and negative cells. Addition of DNA from position -4100 to -5500 causes a reduction in expression to near-basal levels in c-Kit-n egative cells but has little effect in c-Kit-positive cells. The DNA f rom -4100 to -5500 was tested for repressor function. It inhibits tran scription from some heterologous promoters in c-Kit-negative cells, Li kewise, this segment inhibits transcription from the homologous proxim al promoter in a cell-specific manner, but the entire promoter is nece ssary for complete repression in c-Kit-negative cells. Two Myb binding motifs were also identified, and their role in regulating transcripti on was examined by mutation and functional testing. One, MYB1, acts as a partial repressor, whereas the other, MYB2, is a positive element t hat appears essential for expression, Binding proteins to both sites w ere characterized by several methods. MYB1 binds and responds function ally to c-Myb, but MYB2 does not. The results of these studies indicat e that the regulation of c-kit transcription is complex, involving int eractions among several activators and repressors.