Isolation and characterization of two cytochrome P450 cDNA clones for CYP6B6 and CYP6B7 from Helicoverpa armigera (Hubner): possible involvement of CYP6B7 in pyrethroid resistance.

Two new cDNA clones specific for members of the CYP6B gene family, CYP6B6 and CYP6B7 have been isolated from Helicoverpa armigera. The sequences correspond to mRNAs of an estimated 1962 and 2411 nucleotides in length respectively excluding the poly A tails. The two mRNAs have open reading frames encoding proteins of 504 amino acid residues with molecular weights of 57,564 and 58,181 Daltons. Both putative proteins contain the conserved cysteine and surrounding regions characteristics of all cytochrome P450s. The encoded protein sequences show 84-88% protein sequence identity between them and with the previously published cytochrome P450 sequence of H. armigera. CYP6B2. The sequences of cDNA clones of CYP6B6 and CYP6B2 show a very high degree of identity within the first 340 nucleotides which may be the result of a gene conversion event. Two major bands are visible after northern analysis of larval RNA using cDNA clones for CYP6B6, CYP6B7, or the previously published CYP6B2 as probes, due to strong cross-hybridization. Analysis with specific oligonucleotide probes and 3' non-coding regions indicated that the cDNAs for CYP6B6 and CYP6B7 correspond to the smaller and large mRNA bands respectively. The previously identified sequence of CYP6B2, contrary to the previous suggestion, corresponds to a rare mRNA of similar size to that for CYP6B6. The mRNA for CYP6B7 was found to be induced by treatment with the monoterpene, alpha-pinene, and to be over-expressed in some individuals of pyrethroid resistant population of H. armigera. We suggest that CYP6B7 is the form responsible for pyrethroid metabolism in H. armigera.