With aid of a computer, each of the four open reading frames (ORFs) in the P-transposable element of Drosophila melanogaster was aligned with the amino acid sequence of the transposase (TnpA) of bacterial transposon Tn3. The four ORFs, named ORFO, −1, −2, and −3 lined in this order from 5′ to 3′ direction in the P element, turned out to have on the average 18 percent homologies with the Tn3 transposase in the same order without much overlap between adjacent ORFs. Based on the comparison with consensus sequences at exon-intron junctions, three possible introns were predicted in the P element. Each of these putative introns covers the region between adjacent ORFs and these splicings do not alter the initially predicted reading frames. It appears, therefore, that a mature mRNA processed from the transcript of the whole P element codes a single polypeptide having homology to TnpA. This suggests that the P element and the TnpA gene may have diverged from a common ancestor. Taking account of our previous finding that the resolvase of Tn3 has sequence homology with ORF1 of the P element, evolutionary relationships among the P element, the resolvase gene and the TnpA gene were discussed.
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