Isolation and characterization of cDNAs encoding vacuolar H+-pyrophosphatase isoforms from rice (Oryza sativa L)

The vacuolar H+-pyrophosphatase (V-PPase) is an electrogenic H+ pump, which was found in the plant vacuolar membrane. Two cDNA clones (OVP1 and OVP2) encoding the V-PPase were isolated from cultured rice (Oryza sativa L.) cells and subsequently sequenced. The sequence analysis has revealed that OVP1 contains 2316 nucleotides of open reading frame (ORF) and 362 nucleotides of the 3'-untranslated region, whereas OVP2 comprises 2304 nucleotides of ORF and 312 nucleotides of the 3'-untranslated region. The nucleotide sequences of ORF of OVP1 and OVP2 are 80.7% identical, and their 5'- and 3'-untranslated regions have 39.4% and 48.4% identity, respectively. The polypeptides encoded by the ORF of OVP1 and OVP2 contain 771 and 767 amino acids, respectively, and the sequences of the OVP proteins are very similar to those of other V-PPases, which are shown to have 85-91% homology. Chromosomal mapping by RFLP techniques demonstrates that OVP1 and OVP2 are isoforms encoded by different genes. Both OVP1 and OVP2 are mapped on the same chromosome (chromosome 6) to a distance of ca. 90 cM. Northern analysis indicates that the OVP1 and OVP2 are also expressed in intact rice plants and OVP2 shows higher expression in the calli than the roots and shoots, compared to OVP1. These results show that at least two genes encoding the V-PPases are present in rice genome and their expressions are probably regulated in a different manner.