Cloning and characterization of the 5 '-upstream sequence governing the cell cycle-dependent transcription of mouse DNA polymerase alpha 68 kDa subunit gene

We have isolated and determined the structure of the gene encoding the 68 k Da subunit (p68) of the mouse DNA polymerase alpha-primase complex. The p68 gene consists of four exons and the p68 promoter region lacks TATA and CAA T boxes, but contains a GC-rich sequence, two palindrome sequences and two putative E2F-binding sites. A series of transient expression assays using a luciferase reporter gene indicated that a region from nucleotide position -89 to 30 (-89/-30) with respect to the transcription initiation site is cr ucial for basal transcription of the p68 gene in proliferating NIH 3T3 cell s. In particular, part of the GC-rich sequence (-57/-46) and the palindrome (-81/-62) elements were necessary for promoter activity, both of which sha re homology with the E-box sequence. Gel mobility shift assays using NIH 3T 3 nuclear extracts revealed that the upstream stimulatory factor, known as an E-box-binding protein, binds to these sites. Moreover, we observed bindi ng of E2F to two sites near the transcription initiation site (-11/-3 and 9/+16), A transient luciferase expression assay using synchronized NIH 3T3 cells in G(0) phase revealed that these E2F sites are essential for transcr iption induction of the p68 gene after serum stimulation, but are dispensab le for basal transcription. These results indicate that growth-dependent re gulation of transcription of the mouse p68 and p180 genes is mediated by a common factor, E2F; however, basal transcription of the genes, interestingl y, is regulated by different transcription factors.