Effects of light and phytohormones on the expression of cucumber ferrochelatases

Abstract

Ferrochelatase catalyzes the co-ordination of Fe2+ as a central metal into protoporphyrin IX to form protoheme. A ferrochelatase of cucumber (CsFeC1) was localized only in non-photosynthetic tissues [Suzuki et al. (2000) Plant Cell Physiol. 41: 192-199]. In the present study, we isolated a second ferrochelatase cDNA from cucumber (CsFeC2), the amino acid sequence of which showed a relatively low homology with that of CsFeC1. The C-terminal region of CsFeC2 (but not of CsFeC1) showed homology with the conserved LHC motif of light-harvesting chlorophyll-binding protein, and CsFeC2 belonged to a phylogenetic group composed of several plant and cyanobacterial ferrochelatases containing the conserved sequence. It was localized mainly in thylakoid membrane, whereas minor was also detected in the envelope membrane. It was an intrinsic protein bound to the thylakoid membrane and formed complexes, probably with the C-terminal LHC motif. Its mRNA was detected in all tissues and was light responsive in cotyledons, whereas CsFeC1 mRNA was detected in non-photosynthetic tissues and was not light responsive. Cycloheximide treatment in cotyledons had no effect on the level of CsFeC2 mRNA in contrast with the elevation of CsFeC1 mRNA level. Treatment with benzyl adenine increased the level of CsFeC2 mRNA but not of CsFeC1 mRNA. These results suggested that CsFeC2 might mainly involved in the heme biosynthesis in chloroplast. The results of import experiment and measurement of ferrochelatase activity will be reported and discussed in terms of their physiological roles.