The gene encoding p53 mediates a major tumor suppression pathway that is frequently altered in human cancers. p53 function is kept at a low level during normal cell growth and is activated in response to various cellular stresses. The MDM2 oncoprotein plays a key role in negatively regulating p53 activity by either direct repression of p53 transactivation activity in the nucleus or promotion of p53 degradation in the cytoplasm. DNA damage and oncogenic insults, the two best-characterized p53-dependent checkpoint pathways, both activate p53 through inhibition of MDM2. Here we report that the human homologue of MDM2, HDM2, binds to ribosomal protein L11. L11 binds a central region in HDM2 that is distinct from the ARF binding site. We show that the functional consequence of L11-HDM2 association, like that with ARF, results in the prevention of HDM2-mediated p53 ubiquitination and degradation, subsequently restoring p53-mediated transactivation, accumulating p21 protein levels, and inducing a p53-dependent cell cycle arrest by canceling the inhibitory function of HDM2. Interference with ribosomal biogenesis by a low concentration of actinomycin D is associated with an increased L11-HDM2 interaction and subsequent p53 stabilization. We suggest that L11 functions as a negative regulator of HDM2 and that there might exist in vivo an L11-HDM2-p53 pathway for monitoring ribosomal integrity.

Plant peroxidases play a major role in lignin formation and wound healing and are believed to be involved in auxin catabolism and defense to pathogen attack. The function of the anionic peroxidase isozymes is best understood in tobacco. These isozymes catalyze the formation of the lignin polymer and form rigid cross-links between lignin, cellulose, and extensin in the secondary plant cell wall. We report the purification of the anionic peroxidase isozymes from tobacco and their partial amino acid sequence.

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An extracellular, acidic chitinase was purified to homogeneity from tobacco necrosis virus-infected leaves of Cucumis sativis. The amino acid sequences of the intact protein and of peptides isolated following endoproteinase Lys-C digestion, cyanogen bromide cleavage, and trypsin digestion were determined. Oligonucleotide probes derived from this sequence were used to isolate a cDNA clone encoding this protein. No significant homology was found between this chitinase and either the basic chitinase isolated from bean or tobacco or the chitinase isolated from Serratia marcescens; however, strong homology was found between the cucumber chitinase and a lysozyme/chitinase from Parthenocissus quinquifolia. The induction of the protein by tobacco necrosis virus infection or salicylate was found to be at the level of RNA accumulation. Genomic Southern analysis indicates that a single gene in the cucumber genome encodes this protein.Many plants infected with necrotizing pathogens develop local or systemic resistance against subsequent infections (1, 2). This induced resistance is accompanied by a number of biochemical changes in the host plant, including the production of pathogenesis-related (PR) proteins (for review see refs. 3 and 4). The accumulation of these acid-extractable, low molecular weight, protease-resistant proteins correlates with induced resistance, but their function has largely remained elusive. Recently, it has been shown that of the 10 well-characterized PR proteins in tobacco, two (PR-P and -Q) have chitinase activity (5-7), three (PR-O, -N, and -2) have /3-1,3-glucanase activity (8), and two (PR-R and -S) are structurally similar to a maize protease/a-amylase inhibitor (9, 10).From cucumber, we have recently purified one PR protein and identified it as a chitinase (11). After infection with tobacco necrosis virus (TNV), this Mr 28,000 protein accumulates in the intercellular space of the infected, as well as the uninfected, parts of the plant (11, 12). Here, we report the purification of this protein to homogeneity and the determination of 55% of the amino acid sequence (148 of 267 residues) of the mature protein. Oligonucleotide probes were synthesized based on the protein sequence analysis and used to isolate cDNA clones encoding this protein from a library constructed with RNA isolated from TNV-infected cucumber leaves. We have sequenced the cDNA clones and find no significant homology to known chitinase genes. However, striking homology was found between the deduced amino acid sequence and the partial amino acid sequence of a bifunctional lysozyme/chitinase purified from Parthenociccus quinquifolia (13).In preliminary studies on the regulation of chitinase gene expression, we show that there is one gene encoding chitinase in the cucumber genome and the accumulation of this protein after TNV infection or salicylic acid induction correlates with the accumulation of mRNA. MATERIALS AND METHODSProtein Purification and Sequencing. Chitinase protein was isolated from TNV-infected cucumber (Cucu...