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Two promoters that map to 5'-sequences of the human p53 gene are differentially regulated during terminal differentiation of human myeloid leukemic cells.

p53 is overexpressed in many transformed cells and expression of the gene is known to alter during terminal differentiation of cells in culture. Through analysis of recombinant vectors expressing the chloramphenicol acetyl transferase (CAT) gene we found that two promoters map to the 5'-portion of the human p53 oncogene. One promoter, p53p1, maps upstream of the non-coding first exon and the second, p53p2, maps within the first intron. By primer extension analysis of cellular RNA from a number of human cell lines, we found that p53p2 is a functional promoter in vivo. In order to test whether differential regulation of these promoters may be correlated with the control of expression of the p53 gene during differentiation, we have measured the activity of the two promoters by their ability to direct expression of the CAT gene during terminal differentiation of the human promyelocytic leukemia cell line HL-60. HL-60 cells stably harboring Epstein-Barr virus-derived recombinant plasmids that express the CAT gene from either p53p1 or p53p2 were induced to undergo terminal differentiation by a variety of chemical inducers to either granulocytes or monocytes and expression of the CAT gene was measured. The results indicate that while expression of p53p1 remained constant, expression from p53p2 was induced 5- to 10-fold during differentiation of these cells to either granulocytes or monocytes. Similarly, the endogenous p53p2 was found to be induced in HL-60 cells undergoing differentiation. Although the product of the p53p2 initiated transcript has not yet been characterized these results indicate that altered regulation of these two promoters may be important in modulating the expression of mRNA from this gene during terminal differentiation.