Identification and characterisation of a bryophyte polyphenol oxidase encoding gene from Physcomitrella patens

Polyphenol oxidases (PPO) are enzymes of secondary plant metabolism that catalyse the oxidation of polyphenols to quinones. Because of their ubiquitous appearance in the plant kingdom, an important role is assumed; however, the exact physiological function of PPOs remains unclear. In this work, the identification, cloning, and characterisation of a bryophyte PPO from the moss Physcomitrella patens is presented. PPO activity from protein extracts was determined polarographically after activation by SDS. Four Physcomitrella ESTs with homologies to known plant PPOs were selected from publicly accessible databases, and PCR experiments demonstrated that they belong to the same gene, named Pp_ppo1. The identified cDNA was found to be 2402 bp long, containing a single open reading frame of 1611 bp encoding for a 536 amino acid protein with a molecular mass of 60.1 kDa. Cloning and sequencing of a genomic part of Pp_ppo1 revealed the presence of a 94-bp intron. The time course of Pp_ppo1 gene expression in liquid culture was monitored by real time RT-PCR, revealing increasing transcription levels until the 4th day, a maximum between the 4th and the 8th day, and decreasing transcription until the 12th day. A comparison of the deduced amino acid sequence of Pp_ppo1 with seed plant PPOs revealed similarities such as the presence of two highly conserved copper-binding domains and a similar pattern of hydrophobic regions, but also differences such as a stronger membrane association and a shorter signal sequence, thus reflecting the phylogenetic distance of Physcomitrella from seed plants.