The human Pim-1 gene is selectively transcribed in different hemato-lymphoid cell lines in spite of a G + C-rich housekeeping promoter.

The expression of the Pim-1 proto-oncogene was studied by using the K562, Daudi, and Jurkat cell lines. In K562, Pim-1 mRNA levels were more than 20-fold higher than in Daudi and 50-fold higher than in Jurkat. Nuclear run-on assay data correlated directly with the steady-state mRNA levels, suggesting that the rate of transcription was responsible for the selective expression of this gene. Furthermore, the half-life of Pim-1 mRNA was shown to be 47 min in K562, 71 min in Daudi, and 35 min in Jurkat. This indicated that selective Pim-1 mRNA expression did not depend on posttranscriptional regulation. Therefore, 1.7 kilobases of the Pim-1 promoter was sequenced and studied in detail. The sequence showed that the region from nucleotide -1 to -873 was G + C rich (71%). Study of promoter deletions defined two major functional regions, a proximal element (nucleotide -104 to -1) and a distal element (nucleotide -427 to -336). DNase I protection assays identified binding sites for the Sp1 and AP2 proteins in these elements. A possible new transcription factor binds at position -348 in the distal element. In our study of the 1.7-kilobase Pim-1 promoter, we found no differences between K562 and Jurkat that could explain large differences in transcription. Therefore, the Pim-1 promoter appears to function constitutively, and we conclude that distant elements must regulate the tissue-selective expression of this gene. Although the Pim-1 gene has a G + C-rich housekeeping promoter, expression is carefully regulated at the level of transcription.