The RNase PD2 gene of almond (Prunus dulcis) represents an evolutionarily distinct class of S-like RNase genes

A cDNA for an S-like RNase (RNase PD2) has been isolated from a pistil cDNA library of Prunus dulcis cv. Ferragnes. The cDNA encodes an acidic protein of 226 amino acid residues with a molecular weight of 25 kDa. A potential N-glycosylation site is present at the N-terminus in RNase PD2. A signal pe ptide of 23 amino acid residues and a transmembrane domain are predicted. T he two active-site histidines present in enzymes of the T2/S RNase superfam ily were detected in RNase PD2. Its amino acid sequence shows :71.2% simila rity to RNS1 of Arabidopsis and RNase T2 of chickpea, respectively. Norther n blotting and RT-PCR analyses indicate that PD2 is expressed predominantly in petals, pistils of open flowers and leaves of the almond tree. Analyses of shoots cultured in vitro suggested that the expression of RNase PD2 is associated with phosphate starvation. Southern analysis detected two sequen ces related to RNase PD2 in the P. dulcis genome. RFLP analysis showed that S-like RNase genes are polymorphic in different almond cultivars. The PD2 gene sequence was amplified by PCR and two introns were shown to interrupt the coding region. Based on sequence analysis, we have defined three classe s of S-like RNase genes, with the PD2 RNase gene representing a distinct cl ass. The significance of the structural divergence of S-like RNase genes is further discussed.