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<jats:p>Pin1 is an essential and conserved mitotic peptidyl-prolyl isomerase (PPIase) that is distinct from members of two other families of conventional PPIases, cyclophilins and FKBPs (FK-506 binding proteins). In response to their phosphorylation during mitosis, Pin1 binds and regulates members of a highly conserved set of proteins that overlaps with antigens recognized by the mitosis-specific monoclonal antibody MPM-2. Pin1 is here shown to be a phosphorylation-dependent PPIase that specifically recognizes the phosphoserine-proline or phosphothreonine-proline bonds present in mitotic phosphoproteins. Both Pin1 and MPM-2 selected similar phosphorylated serine-proline–containing peptides, providing the basis for the specific interaction between Pin1 and MPM-2 antigens. Pin1 preferentially isomerized proline residues preceded by phosphorylated serine or threonine with up to 1300-fold selectivity compared with unphosphorylated peptides. Pin1 may thus regulate mitotic progression by catalyzing sequence-specific and phosphorylation-dependent proline isomerization.</jats:p>

<jats:p>Nucleosomes, the nucleohistone subunits of chromatin, are present on transcribed eukaryotic genes but do not prevent transcription. It is shown here that the large yeast RNA polymerase III transcribes through a single nucleosome. This takes place through a direct internal nucleosome transfer in which histones never leave the DNA template. During this process, the polymerase pauses with a pronounced periodicity of 10 to 11 base pairs, which is consistent with restricted rotation in the DNA loop formed during transfer. Transcription through nucleosomes by the eukaryotic enzyme and by much smaller prokaryotic RNA polymerases thus shares many features, reflecting an important property of nucleosomes.</jats:p>

<jats:p> Plant disease resistance ( <jats:italic>R</jats:italic> ) genes confer an ability to resist infection by pathogens expressing specific corresponding avirulence genes. In <jats:italic>Arabidopsis thaliana</jats:italic> , resistance to both bacterial and fungal pathogens, mediated by several <jats:italic>R</jats:italic> gene products, requires the <jats:italic>NDR1</jats:italic> gene. Positional cloning was used to isolate <jats:italic>NDR1</jats:italic> , which encodes a 660–base pair open reading frame. The predicted 219–amino acid sequence suggests that NDR1 may be associated with a membrane. <jats:italic>NDR1</jats:italic> expression is induced in response to pathogen challenge and may function to integrate various pathogen recognition signals. </jats:p>

<jats:p> Caspases are a family of cysteine proteases implicated in the biochemical and morphological changes that occur during apoptosis (programmed cell death). The loop domain of Bcl-2 is cleaved at Asp <jats:sup>34</jats:sup> by caspase-3 (CPP32) in vitro, in cells overexpressing caspase-3, and after induction of apoptosis by Fas ligation and interleukin-3 withdrawal. The carboxyl-terminal Bcl-2 cleavage product triggered cell death and accelerated Sindbis virus–induced apoptosis, which was dependent on the BH3 homology and transmembrane domains of Bcl-2. Inhibitor studies indicated that cleavage of Bcl-2 may further activate downstream caspases and contribute to amplification of the caspase cascade. Cleavage-resistant mutants of Bcl-2 had increased protection from interleukin-3 withdrawal and Sindbis virus–induced apoptosis. Thus, cleavage of Bcl-2 by caspases may ensure the inevitability of cell death. </jats:p>