Nucleotide sequence of a cationic peroxidase gene from the tropical forage legume Stylosanthes humilis.

Abstract

Peroxidases (EC 1.1 1.1.7; donor: hydrogen-peroxide oxidoreductase) have been implicated in numerous physiological processes of potential importance in plant-pathogen interactions, including lignification (Walter, 1992), crosslinking of cell-wall components (Bradley et al., 1992), wound healing (Sherf et al., 1993), auxin oxidation (Grambow and Langenbeck-Schwich, 1979), and systemic acquired resistance responses (Irving and Kuc, 1990). A complete description of peroxidase activity in vivo is not available because multiple isoenzymes exist with large numbers of potential substrates. A cDNA (Shpx6, EMBL accession No. L36110) encoding a putative cationic peroxidase was isolated from Stylosanthes humilis. Transcripts hybridizing to this cDNA were shown to be expressed in young leaves and roots of mature plants and were strongly induced in young leaves as a response to wounding and infection with the funga1 phytopathogen Colletotrichton gloeosporioides, which causes anthracnose disease in Stylosanthes spp (Harrison et al., 1994). Southern hybridization analysis with genomic DNA of S. humilis indicated that there are probably two closely related copies of this cationic peroxidase gene (Curtis et al., 1995). We report the isolation of one of the cationic peroxidase genes from S. humilis. A single cationic peroxidase gene with 83% nucleotide sequence homology in the coding region to the cDNA ShpxG was isolated from a genomic DNA library of S. humilis constructed in the A phage EMBL3. The gene encodes a peroxidase proprotein with a putative 25 amino acid signal sequence. The predicted mature protein is 26.2 kD with a pI of 8.71 and contains the three peroxidase signature sequences at amino acid positions 56 to 73, 114 to 151, and 184 to 194 (Buffard et al., 1990). The predicted mature protein also contains the eight Cys residues involved in disulfide bridges and the two His residues involved with binding heme, both of which are characteristic of plant peroxidases (Morgens et al., 1990). The deduced amino acid sequence of this clone, AShpxG, described herein (Table I) has 80% homology to the deduced amino acid sequence of the cDNA Shpx6 and 90 and 48% homology to protein sequences of the peanut peroxidase Pncl (Buffard et al., 1990) and wheat peroxidase POXl (Hertig et al., 1991), respectively. The hShpx6 gene sequence reported is 1848 nucleotides long with two introns