Immune cascade of Spodoptera litura: Cloning, expression, and characterization of inducible prophenol oxidase

Haemolymph associated phenol oxidase is a critical component of invertebrate immune reaction and cuticle sclerotization. Phenol oxidase catalyses the conversion of mono-phenols to diphenols and quinones which finally leads to melanin formation. We have cloned the c-DNA encoding phenol oxidase from the haemocytes of Spodoptera litura and expressed it in Escherichia coli. The encoding gene is 2452 bp with an open reading frame of 2091 bp translating into a 697 amino acid protein. Multiple alignment analysis of the predicted protein sequence shows close homology to other lepidopeteran PPOII type genes. The transcription of the gene is induced upon microbial challenge of 6th instar larvae with E. coli and is unresponsive to injury. Cloning of the ORF of SLPPO in-frame in the E. coli expression vector pQE30 resulted in its expression. Enzymatic analysis of the recombinant protein reveals that the recombinant protein is catalytically active on 4-methyl pyrocatechol upon activation by cetyl pyridinium chloride.