Phosphoenolpyruvate carboxylase isoforms from an aquatic monocot: structure/function relationships

Abstract

Growth at low [CO2] induces a change from C3 to C4 photosynthesis in the aquatic angiosperm, Hydrilla verticillata. A unique feature of this system is that Kranz anatomy is lacking, and both the C4 and Calvin cycles operate within the same cell. Phosphoenolpyruvate carboxylase (PEPC) is the initial carboxylase in the C4 system, and it also has anapleurotic functions in both C3 and C4 plants. Three full-length PEPC cDNA sequences were isolated from Hydrilla, two from leaf tissue and one from root, which showed 95% to 99% homology among the three isoforms. The objective of this study was to identify candidate residues in the isoforms that potentially define characteristic C3 and C4 kinetic and regulatory behavior. Three-dimensional models of the deduced amino acids were constructed using co-ordinates of the E. coli model (Kai et al 1999). Comparative analyses between these and that of the C4 PEPC from Flaveria trinervia were undertaken. None of the three Hydrilla isoforms had the C4 signature Ser, although differential regulation of expression studies indicated that one, Hvpepc4, was the photosynthetic isoform. Most of the substitutions and deletions found in this isoform were outside the known conserved regions, but they still might contribute towards functional variation. Identification of these residues will provide information for site directed mutagenesis experiments to elucidate the structure/function relationship of Hydrilla PEPCs in C3 and C4 photosynthesis.