Independent 5' and 3'-end determination of multiple dihydrofolate reductase transcripts

Multiple dihydrofolate reductase (dhfr) mRNAs, differing substantially in abundance, are produced as a result of the utilization of multiple transcription initiation sites and multiple polyadenylation sites. We have shown that dhfr mRNAs initiating from an upstream promoter region utilize the same collection of six polyadenylation sites and generate multiple dhfr mRNAs at the same relative abundance as do the mRNAs initiating from the major transcription promoter region. These results indicate that the 5' and 3' ends of dhfr mRNAs are independently determined. We show that the relative abundance of steady-state dhfr mRNAs was the same in nuclear and cytoplasmic RNA fractions. This finding makes it unlikely that differences in mRNA stability account for differences in the relative abundance of the multiple dhfr mRNAs in the cytoplasm. Our analysis of the dhfr promoter region revealed the existence of stable cytoplasmic polyadenylated transcripts complementary to the first 300 nucleotides of the dhfr transcripts initiating from the upstream promoter region. Therefore, the dhfr locus hosts two divergent and partially overlapping genes which share the same promoter region.