Gram-positive bacteria use a wealth of extracellular signaling peptides, so-called autoinducers, to regulate gene expression according to population densities. These ''quorum sensing'' systems control vital processes such as virulence, sporulation, and gene transfer. Using x-ray analysis, we determined the structure of PlcR, the major virulence regulator of the Bacillus cereus group, and obtained mechanistic insights into the effects of autoinducer binding. Our structural and phylogenetic analysis further suggests that all of those quorum sensors that bind directly to their autoinducer peptide derive from a common ancestor and form a single family (the RNPP family, for Rap/NprR/PlcR/PrgX) with conserved features. As a consequence, fundamentally different processes in different bacterial genera appear regulated by essentially the same autoinducer recognition mechanism. Our results shed light on virulence control by PlcR and elucidate origin and evolution of multicellular behavior in bacteria.crystal structure ͉ tetratricopeptide repeats ͉ transcription factor

The ubiquitin-proteasome protein degradation system is involved in many essential cellular processes including cell cycle regulation, cell differentiation, and the unfolded protein response.The anaphase-promoting complex/cyclosome (APC/C), an evolutionary conserved E3 ubiquitin ligase, was discovered 15 years ago because of its pivotal role in cyclin degradation and mitotic progression. Since then, we have learned that the APC/C is a very large, complex E3 ligase composed of 13 subunits, yielding a molecular machine of approximately 1 MDa. The intricate regulation of the APC/C is mediated by the Cdc20 family of activators, pseudosubstrate inhibitors, protein kinases and phosphatases and the spindle assembly checkpoint. The large size, complexity, and dynamic nature of the APC/C represent significant obstacles toward high-resolution structural techniques; however, over the last decade, there have been a number of lower resolution APC/C structures determined using single particle electron microscopy. These structures, when combined with data generated from numerous genetic and biochemical studies, have begun to shed light on how APC/C activity is regulated. Here, we discuss the most recent developments in the APC/C field concerning structure, substrate recognition, and catalysis.

CggR is the transcriptional repressor of the gapA operon encoding central glycolytic enzymes in Bacillus subtilis. Recently, a detailed mechanistic characterization of gapA induction revealed that the binding of fructose-1,6-bisphosphate (FBP) to a low affinity site on CggR (Kd > 100 microM) is responsible for repressor release from the DNA. In addition, this prior work demonstrated that FBP binds to a second high affinity site on the repressor, causing a conformational change in the CggR/DNA complexes, but with no consequence on CggR affinity for its operator DNA. In the present study we have thoroughly analyzed the structural and thermodynamic consequences of FBP binding to CggR. Results of fluorescence anisotropy titrations, calorimetry and limited proteolysis confirm the existence in CggR of a high affinity site for FBP, with a Kd of around 6 microM. Using analytical size-exclusion chromatography, ultracentrifugation as well as fluorescence correlation spectroscopy (FCS) and pressure perturbation, we show that FBP binding at this site reduces the size of the CggR oligomers and induces conformational changes that stabilize the dimer against denaturation. Hence, FBP has a dual role on CggR structure and regulatory function. In addition to acting as an inducer of transcription at the low affinity site, FBP bound to the high affinity site acts as a structural cofactor for the repressor, with profound effects on its quaternary structure as well as on its conformational dynamics and stability. This high affinity FBP site apparently evolved from the sugar substrate binding site of homologous enzymes.