Gingipains are cysteine proteinases acting as key virulence factors of the bacterium Porphyromonas gingivalis , the major pathogen in periodontal disease. The 1.5 and 2.0 Å crystal structures of free and D‐Phe‐Phe‐Arg‐chloromethylketone‐inhibited gingipain R reveal a 435‐residue, single‐polypeptide chain organized into a catalytic and an immunoglobulin‐like domain. The catalytic domain is subdivided into two subdomains comprising four‐ and six‐stranded β‐sheets sandwiched by α‐helices. Each subdomain bears topological similarities to the p20‐p10 heterodimer of caspase‐1. The second subdomain harbours the Cys‐His catalytic diad and a nearby Glu arranged around the S1 specificity pocket, which carries an Asp residue to enforce preference for Arg‐P1 residues. This gingipain R structure is an excellent template for the rational design of drugs with a potential to cure and prevent periodontitis. Here we show the binding mode of an arginine‐containing inhibitor in the active‐site, thus identifying major interaction sites defining a suitable pharmacophor.

The genome of Pleurotus ostreatus, an important edible mushroom and a model ligninolytic organism of interest in lignocellulose biorefineries due to its ability to delignify agricultural wastes, was sequenced with the purpose of identifying and characterizing the enzymes responsible for lignin degradation. Heterologous expression of the class II peroxidase genes, followed by kinetic studies, enabled their functional classification. The resulting inventory revealed the absence of lignin peroxidases (LiPs) and the presence of three versatile peroxidases (VPs) and six manganese peroxidases (MnPs), the crystal structures of two of them (VP1 and MnP4) were solved at 1.0 to 1.1 Å showing significant structural differences. Gene expansion supports the importance of both peroxidase types in the white-rot lifestyle of this fungus. Using a lignin model dimer and synthetic lignin, we showed that VP is able to degrade lignin. Moreover, the dual Mn-mediated and Mn-independent activity of P. ostreatus MnPs justifies their inclusion in a new peroxidase subfamily. The availability of the whole POD repertoire enabled investigation, at a biochemical level, of the existence of duplicated genes. Differences between isoenzymes are not limited to their kinetic constants. Surprising differences in their activity T₅₀ and residual activity at both acidic and alkaline pH were observed. Directed mutagenesis and spectroscopic/structural information were combined to explain the catalytic and stability properties of the most interesting isoenzymes, and their evolutionary history was analyzed in the context of over 200 basidiomycete peroxidase sequences. The analysis of the P. ostreatus genome shows a lignin-degrading system where the role generally played by LiP has been assumed by VP. Moreover, it enabled the first characterization of the complete set of peroxidase isoenzymes in a basidiomycete, revealing strong differences in stability properties and providing enzymes of biotechnological interest.

Control of membrane permeability is a key step in regulating the intracellular concentration of antibiotics. Efflux pumps confer innate resistance to a wide range of toxic compounds such as antibiotics, dyes, detergents, and disinfectants in members of the Enterobacteriaceae . The AcrAB-TolC efflux pump is involved in multidrug resistance in Enterobacter cloacae . However, the underlying mechanism that regulates the system in this microorganism remains unknown. In Escherichia coli , the transcription of acrAB is upregulated under global stress conditions by proteins such as MarA, SoxS, and Rob. In the present study, two clinical isolates of E. cloacae , EcDC64 (a multidrug-resistant strain overexpressing the AcrAB-TolC efflux pump) and Jc194 (a strain with a basal AcrAB-TolC expression level), were used to determine whether similar global stress responses operate in E. cloacae and also to establish the molecular mechanisms underlying this response. A decrease in susceptibility to erythromycin, tetracycline, telithromycin, ciprofloxacin, and chloramphenicol was observed in clinical isolate Jc194 and, to a lesser extent in EcDC64, in the presence of salicylate, decanoate, tetracycline, and paraquat. Increased expression of the acrAB promoter in the presence of the above-described conditions was observed by flow cytometry and reverse transcription-PCR, by using a reporter fusion protein (green fluorescent protein). The expression level of the AcrAB promoter decreased in E. cloacae EcDC64 derivates deficient in SoxS, RobA, and RamA. Accordingly, the expression level of the AcrAB promoter was higher in E. cloacae Jc194 strains overproducing SoxS, RobA, and RamA. Overall, the data showed that SoxS, RobA, and RamA regulators were associated with the upregulation of acrAB , thus conferring antimicrobial resistance as well as a stress response in E. cloacae . In summary, the regulatory proteins SoxS, RobA, and RamA were cloned and sequenced for the first time in this species. The involvement of these proteins in conferring antimicrobial resistance through upregulation of acrAB was demonstrated in E. cloacae .