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Insect phenoloxidase (PO) is a typE-Ⅲ copper protein that catalyzes the formation of melanin.Melanin is used in cuticle and eggshell formation,wound healing and to encase and kill invading pathogens.Insect POs are usually first expressed as prophenoloxidases (PPOs),which are activated by processes such as proteolysis.Mosquitoes contain an especially large number of PPOs as compared with other insects.Phyletic distribution analysis confirmed that Aedes aegypti contains one PPO that is closely related to PPOs of non:mosquito insect species; however,the rest of its PPOs are quite mosquito specific.Expression and subsequent characterization of several selectedAe,aegypti PPOs determined that the PPO sharing high sequence identity to PPOs from non:mosquito species is a catecholoxidasE-type PPO,with no detectable hydroxylation activity to monophenolic compounds.The remaining PPOs that are mosquito specific,resemble tyrosinasE-type PPOs that are capable of both hydroxylating p:monophenolic compounds and oxidizing o:diphenolic compounds.Studies of PPOs from Drosophila melanogaster verified the presence of both types of PPOs in this fly species.Based on available PPO sequences in the databases,tyrosinasE-type PPOs seem to be present primarily in dipteran species.Structural analysis and sitE-directed mutagenesis studies revealed that a small,9:10 residue variable region in mosquito PPOs plays a critical role in substrate specificity towards monophenolic compounds.