MAP kinases ͉ Ca ϩ2 ͉ cADP ribose ͉ cGMP ͉ disease resistance

NPR1 is a critical component of the salicylic acid (SA)-mediated signal transduction pathway leading to the induction of defense genes, such as the pathogenesis-related (PR)-1 gene, and enhanced disease resistance. Using a yeast two-hybrid screen, we identified several NPR1-interacting proteins (NIPs). Two of these NIPs are members of the TGA/OBF family of basic leucine zipper (bZIP) transcription factors; this family has been implicated in the activation of SA-responsive genes, including PR-1. Six TGA family members were tested and shown to differentially interact with NPR1: TGA2 and TGA3 showed strong affinity for NPR1; TGA5 and TGA6 exhibited weaker affinity; and TGA1 and TGA4 displayed little or no detectable interaction with NPR1, respectively. Interestingly, the amino-termini of these factors were found to decrease their stability in yeast and differentially affect their apparent affinity toward NPR1. The interacting regions on NPR1 and the TGA factors were also defined. Each of four point mutations in NPR1 that disrupt SA signaling in Arabidopsis completely blocked interaction of NPR1 with TGA2 and TGA3. TGA2 and TGA3 were also found to bind the SA-responsive element of the Arabidopsis PR-1 promoter. These results directly link NPR1 to SA-induced PR-1 expression through members of the TGA family of transcription factors.

We have cloned and sequenced the cDNA and the gene coding for plastid ribosomal protein L4 (RPL4) from two higher plant species, spinach and Arabidopsis thaliana. Ribosomal protein L4 is one of the ribosomal proteins for which extraribosomal functions in transcriptional regulation has been demonstrated in prokaryotes. Sequence comparison of the two plant cDNAs and genes shows that the RPL4 gene has acquired a remarkable 3 extension during evolutionary transfer to the nuclear genome. This extension harbors an intron and codes for a glutamic and aspartic acid-rich amino acid sequence that resembles highly acidic C-terminal tails of some transcription factors. Co-purification of ribosomal protein L4 with plastid RNA polymerase and transcription factor CDF2 using different purification protocols as well as the surprising amino acid sequence of the L4 protein make it a likely candidate to play a role in plastid transcriptional regulation.Higher plant plastid ribosomes are closely related to those found in eubacteria, reflecting the endosymbiotic origin of chloroplasts. They contain about 54 -75 ribosomal proteins, depending on the plant species (1-3). The complete sequencing of several higher plant plastid genomes has shown that about one third of the plastid ribosomal (r) 1 proteins are encoded by the plastid genome itself (4 -7). The remaining two thirds are probably encoded by the nuclear genome, and it is thought that the genes have been transferred from the plastid genome to the nucleus during evolution. From these nuclear-encoded plastid r-proteins, only few have been characterized by their cDNAs and/or by their genes (see Harris et al. (1) and references therein).During the last 10 years ribosomal proteins became of special interest because it was shown in a number of cases that they have extraribosomal functions apart from the ribosome and protein biosynthesis (for review, see Wool (8)). These extraribosomal functions concern basic cellular processes like replication, transcription, RNA processing, translation, and DNA repair. Specific functions in transcription have been reported for r-proteins S10 (9 -12), L4 (13-15) and S14 (16). In all three cases, the regulation by r-proteins concerns the expression of ribosomal components (rRNA, operon S10, and mRNA for rprotein S14).The plastid genome is transcribed by two different RNA polymerases. One is nuclear-encoded, T7-like, and especially active during early phases of plastid development. This polymerase is more or less specified for transcription of plastid housekeeping genes, encluding subunits of the second RNA polymerase, which is plastid-encoded, prokaryotic-like, and transcribes preferentially photosynthesis-related genes during later phases of plastid development (17-21). The transcriptional activity of the prokaryotic-type RNA polymerase is regulated by nuclear-encoded, prokaryotic-type, and sigma-like factors (22, 23).Our group has been working for several years on the expression of rrn transcription in spinach plastids. Transcription of the rrn opero...

SummaryTranscription factors often belong to multigene families and their individual contribution in a particular regulatory network remains difficult to assess. We show here that specific members from a family of conserved Arabidopsis bZIP transcription factors, the TGA proteins, are regulated in their protein stability by developmental stage-specific proteolysis. Using GFP fusions of three different Arabidopsis TGA factors that represent members of distinct subclasses of the TGA factor family, we demonstrate that two of these TGA proteins are specifically targeted for proteolysis in mature leaf cells. Using a supershift gel mobility assay, we found evidence for similar regulation of the cognate proteins as compared to the GFP fusion proteins expressed under the cauliflower mosaic virus (CaMV) 35S promoter. Using various inhibitors, we showed that the expression of at least one of these three TGA factors could be stabilized by inhibition of proteasome-mediated proteolysis. This study indicates that TGA transcription factors may be regulated by distinct pathways of targeted proteolysis that can serve to modulate the contribution of specific members of a multigene family in complex regulatory pathways.