Complete structure of the gene encoding an immunodominant antigen of Dirofilaria immitis and larva-specific synthesis of primary transcript

The complete gene encoding an immunodominant antigen of Dirofilaria immitis was isolated from a Charomid 9–36 genomic DNA library. This genomic DNA clone termed ‘Dg2’ was characterized by restriction mapping, DNA sequencing of the 5′ flanking region, the exon/intron boundaries and the polyadenylation addition site. The Dg2 with 4872 bp in length consisted of five exons interspersed with four introns. These exons reveal a single open reading frame followed by a long 3′ non-coding region of 1383 bp. The open reading frame of 969 bp encodes a polypeptide of 322 amino acids with a molecular weight of 34,400. The ATG translation initiation codon starts 22 nucleotides downstream from the 5′ end of the first exon. The polyadenylation signal sequence, AATAAA, is located at the 3′ end of the last exon. The transcription initiation site was determined by primer extension technique. S1 nuclease mapping analysis demonstrated that the primary transcript derived from Dg2 is synthesized in microfilariae but not in male or female adult worms. The result suggests that the stage-specific expression of Dg2 is regulated at the level of primary transcript.