Tomato Acid Phosphatase-1 Gene from a Nematode Resistant Cultivar

Abstract

The root-knot nematode resistance gene (Mi) in tomato has been shown to be closely linked to an allele encoding an acid phosphatase1 isozyme variant (Apase11, EC 3.1.3.2) (3, 6). Because Mi has no known gene product, there has been considerable interest in isolating the gene for the acid phosphatase enzyme (aps]) as it may provide a means for isolating the nematode resistance gene. One suggested approach is to initiate chromosome walking to Mi using a probe specific for aps] (1, 2, 10). The distance between Mi and aps] has been estimated to be <500 kb2; however, more recent studies suggest that the distance may be as great as 2000 kb (3). Even at the latter distance, isolation of Mi is possible with current methodologies using yeast artificial chromosome libraries and/or chromosome-jumping and -linking libraries. Previous reports have described the isolation and characterization of the Apase1 enzyme, determination of the amino acid sequence of isolated peptide fragments (1, 2, 9, 10), and the isolation and nucleotide sequence of a corresponding cDNA (2, 10). Additionally, these studies have shown that there is one copy of the aps] gene in the tomato genome and that probes specific for aps] can distinguish genotypes, at that locus, by the detection of restriction length polymorphisms on Southern blots (1, 2, 10). Here, we report the complete nucleotide sequence of apsl (Fig. 1). The coding region of the gene spans 2721 base pairs and is interrupted by two introns. The sequence is in full agreement with that reported for the cDNA clones, pLeap2 (2) and pAplb (10), except at position 3531 (Fig. 1), which we have read as a G residue and is read as a T for pAp lb (10). It is likely that this discrepancy is the result of a cloning artifact or a reading error. Other features of the Apase-1 enzyme and the gene sequence are listed in Table I. One other notable feature of the Apase-1 enzyme is its relationship to a class of VSP that have been described in soybeans (5, 7, 8). At the amino acid sequence level, the two characterized soybean VSPs have about 45% homology with the sequence of Apase-1 (2, 10). Comparison of one of the VSP genes (5) to aps] shows that both are comprised of three exons and that the intron/exon junctions for the intervening sequences occur at identical positions in the coding sequence. This provides further evidence that these genes correspond to a closely related family of acid phosphatases that evolved from a common ancestor.