Transcripts from a mosquito dihydrofolate reductase gene: evidence for heterogeneity at the 5' end

Heterogeneity among transcripts from the mosquito (Aedes albopictus) dihydrofolate reductase (DHFR) gene in wild-type C7-12 cells and in methotrexate-resistant Mtx-5011-128 cells has been analyzed by Northern blotting, RNAase mapping, and primer extension. In both sensitive and resistant cells, a major transcription initiation site mapped c. 11 nucleotides downstream of the TATAA box, near position -66 relative to the AUG codon. Two other major transcription initiation sites mapped approx. eight and 45 nucleotides, respectively, upstream of the TATAA box. In addition, at least six minor sites were detected, four of which mapped within TATA-like sequences. Within the AT-rich region flanking the 5'-end of the mosquito DHFR gene were four T-rich motifs and three "GTTTGTG" repeats. Additional "GTTTGTG" repeats occurred in the first exon and in the single 56 nucleotide intron of the mosquito DHFR gene. In contrast to the heterogeneity at the 5'-ends of mosquito DHFR transcripts, the 3'-end terminated at a single position, c. 22 nucleotides downstream of the polyadenylation signal.