An estimate of the number of serine protease genes expressed in sheep blowfly larvae (Lucilia cuprina)

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Abstract
A large and diverse family of serine protease genes was identified in first‐instar larval cDNA of the sheep blowfly (Lucilia cuprina). This complex repertoire of genes was identified via a PCR approach using highly degenerate primers based on structurally conserved regions which surround the active site His and Ser residues found in all serine proteases. PCR products from entire first‐instar larval cDNA, or from third‐instar larval salivary glands or cardia, generated using a microscale RT‐PCR method, were cloned into a plas‐mid vector. Comparison of the restriction fragment patterns of PCR products generated from the three different sources suggests a highly diverse tissue‐specific pattern of serine protease expression in this organism. Detailed analysis of the restriction fragment patterns of sixty‐nine randomly selected clones from entire first‐instar larvae revealed forty‐nine different classes of PCR product. Maximum likelihood analysis of these data indicate that between 125 and 220 different serine protease genes are expressed in first‐instar larvae of L. cuprina. DNA sequence analysis of ten randomly‐selected clones, derived from the three tissue sources, indicated that all ten encoded serine protease gene fragments. A frequently occurring PCR product, generated from both first‐instar total cDNA and third‐instar cardia cDNA, showed 73% amino acid identity to a digestive protease expressed in Droso‐phila melanogaster larval gut cells.