Molecular characterization of the cDNA encoding prophenoloxidase-2 (PPO2) and its expression in diamondback moth Plutella xylostella

Phenoloxidase (PO) is a key enzyme in the development process of insects. The cDNA of Plutella xylostella prophenoloxidase-2 (PxPPO2) was cloned by means of reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The full-length cDNA consisted of 2546bp, containing an open reading frame (ORF) of 2091bp which encoding 696 amino acids. BLASTp search and neighbor-joining analysis showed that the deduced amino acid sequence had a high identity to the published sequence of PPO2 from other lepidopterous insects, ranging from 72.2% to 78.3%. Semi-quantitative RT-PCR and real-time PCR indicated that the highest amount of PxPPO2 transcripts in eggs or the 4th instar larvae, followed by the 2nd, the 3rd instar larvae, prepupae, and pupa. Real-time PCR analysis showed that when the 3rd instar larvae treated with quercetin or kojic acid, the transcript levels of PPO2 became significantly decreased as compared with control ones.