cDNA sequence and expression of subunit E of the vacuolar H+-ATPase in the inducible Crassulacean acid metabolism plant Mesembryanthemum crystallinum

A cDNA coding for subunit E of the vacuolar H+-ATPase was cloned from Mesembryanthemum crystallinum, a plant which switches from C3-photosynthesis to Crassulacean acid metabolism under saline growth conditions. Sequence homology between the three subunit E-polypeptides of different higher plant species varied between 77.6 and 73.3%; peptide length was between 226 and 230 amino acid residues, 43 of which are invariant in all seven subunit E-polypeptides known so far from animals, fungi and plants. The deduced relative molecular mass of subunit E in Mesembryanthemum crystallinum is 26 162 Da. Subunit E is present both in C3- and CAM-plants. mRNA levels increased severalfold in leaves of CAM-induced plants. This was accompanied by a less pronounced increase in subunit E protein. Obviously, expression is stimulated under conditions of increased requirement for tonoplast H+-pumping activity.