Structural Basis of the Substrate Specificity and Enzyme Catalysis of a Papaver somniferum Tyrosine Decarboxylase
Dopa decarboxylase (DDC) functions in insect catecholamine biochemistry to produce materials essential for cross-linking reactions that result in tanning and/or melanization, include tanning of the mosquito egg chorion and encapsulation of parasites. We have cloned Ddc from the mosquito, Aedes aegypti, and studied its expression in response to blood-feeding, which initiates events necessary for egg maturation in mosquitoes. The Ae. aegypti Ddc cDNA was isolated via heterologous screening using a clone from Drosophila melanogaster. A resulting 1.87 kilobase (kb) clone was sequenced to reveal an open reading frame of 1464 bp, as well as 5'- and 3'-untranslated segments. The inferred amino acid sequence of this clone shares 81% identity with the published Drosophila Ddc cDNA, including complete identity with twenty-four contiguous amino acids encompassing the pyridoxal-5-phosphate cofactor binding domain. Analysis of an F2 intercross population derived from a parental cross between two Ae. aegypti strains (Hamburg and Moyo-In-Dry) allowed us to map Ddc to a locus on linkage group 2. Expression studies demonstrated the presence of a 2.1 kb message, the majority of which occurs in the ovaries where Ddc-specific mRNA is up-regulated in response to ingestion of a blood meal. The potential for egg-tanning in anautogenous mosquitoes as a model for understanding specific genetic events in the regulation of catecholamine metabolism is addressed.