IDENTIFICATION AND MOLECULAR-CLONING OF 2 HOMOLOGS OF PROTEIN PHOSPHATASE-X FROM ARABIDOPSIS-THALIANA

In a recent paper [Arino et al., Plant Mol Biol 21: 475-485 (1993)] we reported the amplification of a DNA fragment (AP-2) from the genome o f Arabidopsis thaliana encoding an amino acid sequence corresponding t o a Ser/Thr protein phosphatase distantly related to type 2A protein p hosphatases. In this paper we report the use of the AP-2 fragment to i solate several cDNA clones from a leaf cDNA library. Two of these (EP 124 and EP 129) largely overlap and contain the AP-2 sequence, whereas a third clone (EP128) is different although very related in sequence (86% of identity). Clones EP124/EP129 and EP128 were found to encode t wo highly related polypeptides (93% identity) of 305 residues, showing a very high identity (83%) to the catalytic subunit of protein phosph atase X (PPX) from rabbit. Therefore, they have been named PPX-1 (EP12 4/EP129) and PPX-2 (EP128). Southern blot analysis of genomic DNA indi cates that only these two genes encoding phosphatases closely related to PPX are present in the genome of A. thaliana. Both PPX-1 and PPX-2 are expressed at very low levels in A. thaliana flowers, leaves, stems and roots. The expression levels of four previously identified type 2 A phosphatases are higher than those of PPX genes. PP2A-1 appears to b e the major mRNA species detected in all the tissues analyzed.