cDNA and derived amino acid sequence of rabbit nasal cytochrome P450NMb (P450IIG1), a unique isozyme possibly involved in olfaction

Olfactory-specific cytochrome P450NMb was previously purified to electrophoretic homogeneity from microsomes of rabbit nasal mucosa in this laboratory. In the present study, a cDNA library made from poly(A)⁺ RNA from rabbit nasal mucosa was screened with antibodies to this P450, and eight immunopositive clones were isolated and characterized. The sequence determined from two overlapping clones contained an open reading frame of 1446 nucleotides, with the predicted first 39 amino acids corresponding to residues 12 to 50 of purified NMb, except for position 46, where Leu was encoded instead of the Glu residue that was found earlier by Edman degradation analysis. The complete polypeptide, including residues 1 to 11, contains 494 amino acid residues and has a molecular weight of 56,640. Sequence comparisons indicated that NMb is more than 50% identical to members of the rabbit P450 gene II family, including IIB4, IIC3, IIC5, IIE1, and IIE2, and 83% identical to rat P450olf1 (IIG1). Hybridization of NMb to electrophoretically fractionated rabbit nasal poly(A)⁺ RNA revealed 3.6- and 2.1-kb species, but with a probe derived from the 3′-nontranslated portion of the cDNA only the 3.6-kb band was observed, suggesting the use of alternate polyadenylation sites or splicing. In agreement with the known tissue-specific distribution of NMb protein, NMb transcripts were found in olfactory mucosa, but not in liver, lung, intestine, or kidney. Genomic hybridization analysis indicated that there may be only one copy of the NMb gene present in the rabbit genome.