An amplified insect dihydrofolate reductase gene contains a single intron

We have used methotrexate‐resistant mosquito (Aedes albopictus) cells as the source of DNA for cloning an 8.5‐kb EcoRI fragment containing an amplified dihydrofolate reductase (DHFR) gene. An estimated 1200 copies of the DHFR gene were represented in nuclear DNA from Mtx‐5011‐256 cells, which were 3000‐fold more resistant to methotrexate than wild‐type cells. Southern blot analysis indicated that all of the amplified DHFR genes were contained within a 1.8‐kb AccI fragment represented in the cloned DNA. In contrast to mammalian DHFR genes which span approximately 30 kb, the complete amino acid coding sequence of the mosquito DHFR gene spanned 614 nucleotides, including a single 56‐nucleotide intron that interrupted a conserved Arg codon at amino acid position 27. Additional introns characteristic of mammalian DHFR genes were absent; conservation of the first intron in the mosquito DHFR gene supports a regulatory role for this intron. The mosquito DHFR gene coded for a 186‐amino‐acid protein with 43–48% similarity to vertebrate DHFR.