Quorum sensing is one of several mechanisms that bacterial cells use to interact with each other and coordinate certain physiological processes in response to cell density. This mechanism is mediated by extracellular signaling molecules; once a critical threshold concentration has been reached, a target sensor kinase or response regulator is activated (or repressed), facilitating the expression of quorum sensing-dependent genes. Gram-positive bacteria mostly use oligo-peptides as signaling molecules. These cells have a special kind of quorum-sensing systems in which the receptor protein interacts directly with its cognate signaling peptide. The receptors are either Rap phosphatases or transcriptional regulators and integrate the protein family RNPP, from Rap, Npr, PlcR, and PrgX. These quorum-sensing systems control several microbial processes, like sporulation, virulence, biofilm formation, conjugation, and production of extracellular enzymes. Insights of the mechanism of protein-signaling peptide binding as well as the molecular interaction among receptor protein, signaling peptide, and target DNA have changed some earlier perceptions. In spite of the increased knowledge and the potential biotechnological applications of these quorum-sensing systems, few examples on engineering for biotechnological applications have been published. Real applications will arise only when researchers working in applied microbiology and biotechnology are aware of the importance of quorum-sensing systems for health and bioprocess applications.

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Abstract. We use simultaneous observations of tropospheric ozone and outgoing longwave radiation (OLR) sensitivity to tropospheric ozone from the Tropospheric Emission Spectrometer (TES) to evaluate model tropospheric ozone and its effect on OLR simulated by a suite of chemistry-climate models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The ensemble mean of ACCMIP models show a persistent but modest tropospheric ozone low bias (5–20 ppb) in the Southern Hemisphere (SH) and modest high bias (5–10 ppb) in the Northern Hemisphere (NH) relative to TES ozone for 2005–2010. These ozone biases have a significant impact on the OLR. Using TES instantaneous radiative kernels (IRK), we show that the ACCMIP ensemble mean tropospheric ozone low bias leads up to 120 mW m−2 OLR high bias locally but zonally compensating errors reduce the global OLR high bias to 39 ± 41 m Wm−2 relative to TES data. We show that there is a correlation (R2 = 0.59) between the magnitude of the ACCMIP OLR bias and the deviation of the ACCMIP preindustrial to present day (1750–2010) ozone radiative forcing (RF) from the ensemble ozone RF mean. However, this correlation is driven primarily by models whose absolute OLR bias from tropospheric ozone exceeds 100 m Wm−2. Removing these models leads to a mean ozone radiative forcing of 394 ± 42 m Wm−2. The mean is about the same and the standard deviation is about 30% lower than an ensemble ozone RF of 384 ± 60 m Wm−2 derived from 14 of the 16 ACCMIP models reported in a companion ACCMIP study. These results point towards a profitable direction of combining satellite observations and chemistry-climate model simulations to reduce uncertainty in ozone radiative forcing.

NprR belongs to the RNPP family of quorum-sensing receptors, a group of intracellular regulators activated directly by signaling oligopeptides in Gram-positive bacteria. In Bacillus thuringiensis (Bt), nprR is located in a transcriptional cassette with nprRB that codes for the precursor of the signaling peptide NprRB. NprR is a transcriptional regulator activated by binding of reimported NprRB; however, several reports suggest that NprR also participates in sporulation but the mechanism is unknown. Our in silico results, based on the structural similarity between NprR from Bt and Spo0F-binding Rap proteins from Bacillus subtilis, suggested that NprR could bind Spo0F to modulate the sporulation phosphorelay in Bt. Deletion of nprR-nprRB cassette from Bt caused a delay in sporulation and defective trigger of the Spo0A∼P-activated genes spoIIA and spoIIIG. The DNA-binding domain of NprR was not necessary for this second function, since truncated NprRΔHTH together with nprRB gene was able to restore the sporulation wild type phenotype in the ΔnprR-nprRB mutant. Fluorescence assays showed direct binding between NprR and Spo0F, supporting that NprR is a bifunctional protein. To understand how the NprR activation by NprRB could result in two different functions, we studied the molecular recognition mechanism between the signaling peptide and the receptor. Using synthetic variants of NprRB, we found that SSKPDIVG displayed the highest affinity (Kd = 7.19 nM) toward the recombinant NprR and demonstrated that recognition involves conformational selection. We propose that the peptide concentration in the cell controls the oligomerization state of the NprR-NprRB complex for switching between its two functions.

Amebiasis is one of the most frequent protozoan infections of man, because an estimated 10% of the world population is affected (1). The disease is caused by Entamoeba histolytica, the only Entamoeba known to be pathogenic for humans. The motile form or trophozoite can live in the intestinal lumen as a harmless commensal, but occasionally, and for unknown reasons, it can invade the colonic mucosa giving rise to amebic dysentery and subsequently to liver abscesses (2). In addition to E. histolytica, nonpathogenic Entamoeba species like Entamoeba coli, and Entamoeba hartmanni may inhabit the human intestine. The reasons for the different behavior of pathogenic and nonpathogenic Entamoeba are not known.Basic structural or biochemical differences which could account for the erratic invasive behavior of Entamoeba have not been found (3, 4). However, pathogenic strains of E. histolytica differ from strains isolated from asymptomatic carriers and other nonpathogenic Entamoeba in certain surface properties, such as the susceptibility to agglutinate with the plant lectin concanavalin A, and the lack of surface charge (5, 6).Another surface property characteristic of Entamoeba is the ability to phagocytize a variety of particulate material including starch grains, bacteria, various protozoa, inert polyestyrene beads, and erythrocytes (7-16). More than 100 yr ago Lesh (LiSsch) (17) demonstrated that amebas in stool samples from human dysentery contained erythrocytes. Since then, it has been claimed repeatedly that invasive strains of E. histolytica are the only intestinal amebas of humans able to ingest erythrocytes. Erythrophagocytosis has been traditionally considered as one of the most important criteria in identifying pathogenic E. histolytica trophozoites, as a perusal of texts on human protozoology will reveal (1, 18-25) with few exceptions (2).In view of the importance given to the presence of ingested erythrocytes for the identification of invasive E. histolytica, the lack of systematic comparative studies is striking. On one hand, Entamoeba other than E. histolytica have occasionally been shown to engulf erythrocytes both in vivo and in vitro (16,26,27). On the other hand, studies on E. moshkovskii (the only free-living Entamoeba known) are contradictory, because it has been reported to be able (28) or unable (29) to phagocytize erythrocytes. Shaffer and Ansfield (30) studied the erythrophagocytosis in a number of E. histolytica strains without specifying their virulence.In continuation of our studies on the surface properties of Entamoebae (5, 6) we have