3'-NADP and 3'-NAADP, Two Metabolites Formed by the Bacterial Type III Effector AvrRxo1.

An arsenal of effector proteins is injected by bacterial pathogens into the host cell or its vicinity to increase virulence. The commonly used top-down approaches inferring the toxic mechanism of individual effector proteins from the host's phenotype are often impeded by multiple targets of different effectors as well as by their pleiotropic effects. Here we describe our bottom-up approach, showing that the bacterial type III effector AvrRxo1 of plant pathogens is an authentic phosphotransferase that produces two novel metabolites by phosphorylating nicotinamide/nicotinic acid adenine dinucleotide at the adenosine 3'-hydroxyl group. Both products of AvrRxo1, 3'-NADP and 3'-nicotinic acid adenine dinucleotide phosphate (3'-NAADP), are substantially different from the ubiquitous co-enzyme 2'-NADP and the calcium mobilizer 2'-NAADP. Interestingly, 3'-NADP and 3'-NAADP have previously been used as inhibitors or signaling molecules but were regarded as "artificial" compounds so far. Our findings now necessitate a shift in thinking about the biological importance of 3'-phosphorylated NAD derivatives.

Type II toxin: antitoxin systems. More than small selfish entities?

Toxin-antitoxin (TA) modules regulate metabolism and viability of bacteria and archaea. In type II TA systems these functions are generally thought to be performed by two small proteins. However, evidence is increasing that the toxins are much more diverse and can form multi-domain proteins. Recently, we published a novel type II TA system in which toxin and antitoxin are covalently linked into a single polypeptide chain. In this review we summarize the current knowledge on these elongated toxin homologs and provide perspectives for future study.

A cis-acting antitoxin domain within the chromosomal toxin-antitoxin module EzeT of Escherichia coli quenches toxin activity.

Toxin-antitoxin (TA) systems are widespread genetic modules in the genomes of bacteria and archaea emerging as key players that modulate bacterial physiology. They consist of two parts, a toxic component that blocks an essential cellular process and an antitoxin that inhibits this toxic activity during normal growth. According to the nature of the antitoxin and the mode of inhibition, TA systems are subdivided into different types. Here, we describe the characterization of a type II-like TA system in Escherichia coli called EzeT. While in conventional type II systems the antitoxin is expressed in trans to form an inactive protein-protein complex, EzeT consists of two domains combining toxin and cis-acting antitoxin functionalities in a single polypeptide chain. We show that the C-terminal domain of EzeT is homologous to zeta toxins and is toxic in vivo. The lytic phenotype could be attributed to UDP-N-acetylglucosamine phosphorylation, so far only described for type II epsilon/zeta systems from Gram-positive streptococci. Presence of the N-terminal domain inhibits toxicity in vivo and strongly attenuates kinase activity. Autoinhibition by a cis-acting antitoxin as described here for EzeT-type TA systems can explain the occurrence of single or unusually large toxins, further expanding our understanding of the TA system network.

Single mimivirus particles intercepted and imaged with an X-ray laser.

X-ray lasers offer new capabilities in understanding the structure of biological systems, complex materials and matter under extreme conditions. Very short and extremely bright, coherent X-ray pulses can be used to outrun key damage processes and obtain a single diffraction pattern from a large macromolecule, a virus or a cell before the sample explodes and turns into plasma. The continuous diffraction pattern of non-crystalline objects permits oversampling and direct phase retrieval. Here we show that high-quality diffraction data can be obtained with a single X-ray pulse from a non-crystalline biological sample, a single mimivirus particle, which was injected into the pulsed beam of a hard-X-ray free-electron laser, the Linac Coherent Light Source. Calculations indicate that the energy deposited into the virus by the pulse heated the particle to over 100,000 K after the pulse had left the sample. The reconstructed exit wavefront (image) yielded 32-nm full-period resolution in a single exposure and showed no measurable damage. The reconstruction indicates inhomogeneous arrangement of dense material inside the virion. We expect that significantly higher resolutions will be achieved in such experiments with shorter and brighter photon pulses focused to a smaller area. The resolution in such experiments can be further extended for samples available in multiple identical copies.