Expression and Promoter Activity of Genes for Isozymes of Horseradish Peroxidase : GENE STRUCTURE AND EXPRESSION

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The expression and promoter activity of genes for isozymes of horseradish peroxidase, namely, prxC1a, prxC1b, prxC2 and prxC3, were studied. Organ-specific expression of these genes in horseradish plants was examined by Northern blot analysis. The group of prxC1 genes was expressed mostly in stems, while prxC2 and prxC3 were expressed to a greater extent in roots. Hardly any expression of any of the genes was detected in leaves. In transient-expression assays with tobacco protoplasts, about 500 bp of the 5'-noncoding regions of each of the genes, ligated to the gene for β-glucuronidase (GUS), exhibited significant promoter activity. In particular, the fragments extending from the initiation codon of the prxC2 gene to -529 bp and -1 kbp supported high levels of GUS activity, which were 4.4 and 11.4 times respectively, the activity observed under control of the 35S promoter from cauliflower mosaic virus (CaMV). Conserved enhancer sequences of human genes were found in the 5'-flanking region of prxC2, and deletion of the regions that contained the enhancer sequences reduced the GUS activity. High levels of GUS activity were observed in transgenic tobacco plants that contained I kbp of the 5' flanking region of prxC2 fused to the GUS gene. GUS activity was diminished when deletion from the 5' end extended as far as the CAAT box. No significant organ-specific expression of GUS was observed with any such deletion.
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