AtObgC-AtRSH1 interaction may play a vital role in stress response signal transduction in Arabidopsis

The interaction of Obg (Spo0B-associated GTP-binding protein) GTPase and SpoT, which is a bifunctional ppGpp (guanosine 3',5'-bispyrophosphate) hydrolase/synthetase, is vital for the modulation of intracellular ppGpp levels during bacterial responses to environmental cues. It has been recently reported that the ppGpp level is also inducible by various stresses in the chloroplasts of plant cells. However, the function of the Obg-SpoT interaction in plants remains elusive. The results from the present and previous studies suggest that AtRSH1 is a putative bacterial SpoT homolog in Arabidopsis and that its transcription levels are responsive to wounding and salt stresses. In this study, we used a yeast two-hybrid analysis to map the regions required for the AtObgC-AtRSH1 interaction. Moreover, protein-protein docking simulations revealed reasonable geometric and electrostatic complementarity in the binding surfaces of the two proteins. The data support our experimental results, which suggest that the conserved domains in AtObgC and the N terminus of AtRSH1 containing the TGS domain contribute to their interaction. In addition, quantitative real-time PCR (qRT-PCR) analyses showed that the expression of AtObgC and AtRSH1 exhibit a similar inhibition pattern under wounding and salt-stress conditions, but the inhibition pattern was not greatly influenced by the presence or absence of light. Based on in vivo analyses, we further confirmed that the AtRSH1 and AtObgC proteins similarly localize in chloroplasts. Based on these results, we propose that the AtObgC-AtRSH1 interaction plays a vital role in ppGpp-mediated stress responses in chloroplasts.

Molecular characterization of two glutathione peroxidase genes of Panax ginseng and their expression analysis against environmental stresses

Glutathione peroxidases (GPXs) are a group of enzymes that protect cells against oxidative damage generated by reactive oxygen species (ROS). GPX catalyzes the reduction of hydrogen peroxide (H2O2) or organic hydroperoxides to water or alcohols by reduced glutathione. The presence of GPXs in plants has been reported by several groups, but the roles of individual members of this family in a single plant species have not been studied. Two GPX cDNAs were isolated and characterized from the embryogenic callus of Panax ginseng. The two cDNAs had an open reading frame (ORF) of 723 and 681bp with a deduced amino acid sequence of 240 and 226 residues, respectively. The calculated molecular mass of the matured proteins are approximately 26.4kDa or 25.7kDa with a predicated isoelectric point of 9.16 or 6.11, respectively. The two PgGPXs were elevated strongly by salt stress and chilling stress in a ginseng seedling. In addition, the two PgGPXs showed different responses against biotic stress. The positive responses of PgGPX to the environmental stimuli suggested that ginseng GPX may help to protect against environmental stresses.

Emergence and expansion of TFIIB-like factors in the plant kingdom

Many gene families in higher plants have expanded in number, giving rise to diverse protein paralogs with specialized biochemical functions. For instance, plant general transcription factors such as TFIIB have expanded in number and in some cases perform specialized transcriptional functions in the plant cell. To date, no comprehensive genome-wide identification of the TFIIB gene family has been conducted in the plant kingdom. To determine the extent of TFIIB expansion in plants, I used the remote homology program HHPred to search for TFIIB homologs in the plant kingdom and performed a comprehensive analysis of eukaryotic TFIIB gene families. I discovered that higher plants encode more than 10 different TFIIB-like proteins. In particular, Arabidopsis thaliana encodes 14 different TFIIB-like proteins and predicted domain architectures of the newly identified TFIIB-like proteins revealed that they have unique modular domain structures that are divergent in sequence and size. Phylogenetic analysis of selected eukaryotic organisms showed that most life forms encode three major TFIIB subfamilies that include TFIIB, Brf, Rrn7/TAF1B/MEE12 subfamilies, while all plants and some algae species encode one or two additional TFIIB-related protein subfamilies. A subset of A. thaliana GTFs have also expanded in number, indicating that GTF diversification and expansion is a general phenomenon in higher plants. Together, these findings were used to generate a model for the evolutionary history of TFIIB-like proteins in eukaryotes.