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Cloning and Sequencing of a Second Ribosome-Inactivating Protein Gene from Maize (Zea mays L.)

Toxic proteins are among the wide array of antibiotic compounds produced by plants. Some of the most potent plant toxins belong to a class of translational inhibitors called RIPs (Stirpe et al., 1992). RIPs act as N-glycosidases to remove a specific adenine from the large rRNA, thereby irreversibly inactivating the ribosome. A maize Rip gene encodes a highly expressed seed-specific RIP (also called b-32 or proRIP) that accumulates under the control of the transcriptional activator, Opaque-2, as a nontoxic, singlechain zymogen (Soave et al., 1981; Walsh et al., 1991; Bass et al., 1992). During germination the RIP is proteolytically cleaved to an active form with two noncovalently linked chains of 8 and 16 kD (Walsh et al., 1991). A genomic library was screened with a cDNA clone for the seed RIP. This led to the isolation of a genomic clone for a second, nonallelic Rip gene from maize. The sequence of a 963-bp segment within this clone is described in Table I. This region includes an ORF that predicts a 31-kD polypeptide with 73% amino acid identity with maize seed proRIP. Among the residues shared are the five active site cleft residues found in a11 plant RIPs for which data are available (Bass et al., 1992). Genomic Southern analysis and recombinant inbred restriction fragment length polymorphism mapping results are consistent with a two-member Rip gene family. One member corresponds to the seed-specific Rip gene, which has been previously mapped by segregation analysis of allelic protein isoforms to chromosome 8L (Soave et al., 1981). The Rip gene described here corresponds to a second member that can be placed on chromosome 7L (Bass et al., 1990). The existence of multiple RIPs, and in some cases organ-specific isoforms, has also been observed in severa1 other plant species (Stirpe et al., 1992). We propose that the seed-specific Rip gene be designated Rip3:l for the first member of the maize Rip multigene family and that the gene described here be designated Rip3:2 (Mundy et al., 1994). Alignment of the deduced amino acid sequences of the two maize RIPs reveals little similarity in the regions cor-