A 165-base pair sequence between the dihydrofolate reductase gene and the divergently transcribed upstream gene is sufficient for bidirectional transcriptional activity.

The dihydrofolate reductase gene encodes a key enzyme of one-carbon metabolism and is constitutively expressed in all cells. Recently, transcripts initiated at 89 base pairs upstream from the transcriptional initiation site of the dihydrofolate reductase gene and transcribed from the opposite strand have been identified and shown to encode for a protein with homology to a bacterial DNA mismatch repair enzyme (Fujii, H., and Shimada, T. (1989) J. Biol. Chem. 264, 10057-10064). Therefore, the two genes are organized in a head-to-head configuration separated by an 89-base pair segment. The promoter activities of this short spacer sequence were studied in a transient assay using the chloramphenicol acetyltransferase and the guanine phosphoribosyltransferase genes as reporters. A 165-base pair fragment from -111 to +54 relative to the dihydrofolate reductase initiation site was shown to be sufficient for transcriptional activity in either direction, suggesting that expression of the two divergent genes is regulated by a bidirectional promoter that may use common regulatory elements.