Structure of rabbit butyrylcholinesterase gene deduced from genomic clones and from cDNA with introns

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Abstract
Three clones were isolated from a rabbit genomic library. They covered the entire coding sequence of the rabbit BChE gene. The positions of splice sites between exons 2, 3, and 4 are identical to those found in the human gene (Arpagauset al., 1990). Exon 2 covers 83% of the coding sequence. This contrasts with the small size of exon 3 (167 bp) and large size of introns 2 and 3 (>20 kb each). The active-site serine at position 198 is found in a highly conserved region. Aspartic acids in positions 91 and 170 are conserved in human and rabbit, and one of them could be involved in the calytic triad. Aspartic acid 70, present in the anionic site of human BChE, is also conserved in rabbit BChE. The coding sequences of human and rabbit BChE are 89% identical over 744 bp around the active-site serine. In addition to the genomic clones, one cDNA clone (BNY1) was isolated. This cDNA was unusual in that it contained intronic sequences. The insert of 1 kb contained 167 coding bases homologous to the nucleotide sequence 1434 to 1600 of human cDNA and corresponded to exon 3 of the BChE gene. On each side of this coding region, consensus sequences of intron-exon boundaries were found. The presence of large-size transcripts in Northern blots and the existence of a cDNA copy of unprocessed mRNA found in the BNY1 clone suggest a slow processing of transcripts. A genomic sequence unspliced in a cDNA ofTorpedo AChE could give a transmembrane domain (Sikoravet al., 1988); the corresponding sequence in rabbit BChE gene, also found in a cDNA, had no homology withTorpedo AChE but could be translated in a hydrophobic C-terminal domain if maintained in mature mRNA.