A Bacterial Ras-Like Small GTP-Binding Protein and Its Cognate GAP Establish a Dynamic Spatial Polarity Axis to Control Directed Motility

Regulated cell polarity is central to many cellular processes. We investigated the mechanisms that govern the rapid switching of cell polarity (reversals) during motility of the bacterium Myxococcus xanthus. Cellular reversals are mediated by pole-to-pole oscillations of motility proteins and the frequency of the oscillations is under the control of the Frz chemosensory system. However, the molecular mechanism that creates dynamic polarity remained to be characterized. In this work, we establish that polarization is regulated by the GTP cycle of a Ras-like GTPase, MglA. We initially sought an MglA regulator and purified a protein, MglB, which was found to activate GTP hydrolysis by MglA. Using live fluorescence microscopy, we show that MglA and MglB localize at opposite poles and oscillate oppositely when cells reverse. In absence of MglB, MglA-YFP accumulates at the lagging cell end, leading to a strikingly aberrant reversal cycle. Spatial control of MglA is achieved through the GAP activity of MglB because an MglA mutant that cannot hydrolyze GTP accumulates at the lagging cell end, despite the presence of MglB. Genetic and cell biological studies show that the MglA-GTP cycle controls dynamic polarity and the reversal switch. The study supports a model wherein a chemosensory signal transduction system (Frz) activates reversals by relieving a spatial inhibition at the back pole of the cells: reversals are allowed by Frz-activated switching of MglB to the opposite pole, allowing MglA-GTP to accumulate at the back of the cells and create the polarity switch. In summary, our results provide insight into how bacteria regulate their polarity dynamically, revealing unsuspected conserved regulations with eukaryots.     

La diatomée marine Chaetoceros simplex calcitransPaulsen et son environnement. VI. Capture et métabolisme du cholestérol

An 8-day culture was made of the marine diatom Chaetoceros simplex calcitrans Paulsen in the presence of cholesterol-4 ¹⁴C. The collected cells were then introduced into a replacement medium for a new period of 8 days. The capture and metabolism of the sterol were followed and information was obtained concerning the exchanges between the cells and the medium; for this purpose, hydrocarbons and fatty acids have been analysed. Evidence for the degradation of cholesterol into acetate is established. The observed phenomena are rapid, complex, and apparently modulated at all levels of the exchanges.     

Infochemical-mediated preference behavior of the parasitoid Microctonus hyperodae when searching for its adult weevil hosts

Infochemicals are the most important cues used by parasitoids for host location. The attractiveness of infochemicals in a tritrophic context is expected to be determined by the degree of specialization of the parasitoid and its host(s). Microctonus hyperodae Loan (Hymenoptera: Braconidae) is an oligophagous parasitoid that attacks adult Curculionidae of the Brachycerinae subfamily, especially Listronotus bonariensis Kuschel, on Gramineae. In 1996, a new host–parasitoid association between the carrot weevil Listronotus oregonensis LeConte and M. hyperodae was created in the laboratory. In this study, the infochemicals used by M. hyperodae when searching for its adult weevil hosts were determined using a Y‐shaped olfactometer. Three curculionid species (L. oregonensis, Listronotus sparsus Say, and Neydus flavicaudis Boheman) and one bruchid species [Callosobruchus maculatus (Fabricius)], and their feces, were tested. It was expected that hosts phylogenetically and ecologically close to L. bonariensis would be more attractive than species less related but in fact, M. hyperodae responded only to L. oregonensis and its feces. When feces and host insects were tested separately, M. hyperodae responded to the odors emitted by L. oregonensis adults but not to their feces, suggesting that most of the kairomones came from the host itself. Host plants were also tested, but M. hyperodae responded neither to Lolium multiflorum Lamark (Gramineae) nor to Daucus carota L. (Umbelliferae) leaves.     

Homology between a 173-kb Region from Mouse Chromosome 10, Telomeric to the Ifng Locus, and Human Chromosome 12q15

We sequenced a 173-kb region of mouse chromosome 10, telomeric to the Ifng locus, and compared it with the human homologous sequence located on chromosome 12q15 using various sequence analysis programs. This region has a low density of genes: one gene was detected in the mouse and the human sequences and a second gene was detected only in the human sequence. The mouse gene and its human orthologue, which are expressed in the immune system at a low level, produce a noncoding mRNA. Nonexpressed sequences show a higher degree of conservation than exons in this genomic region. At least three of these conserved sequences are also conserved in a third mammalian species (sheep or cow).     

Two γ-tubulin isoforms are differentially expressed during development in Helianthus annuus

The cytoskeleton is involved in major developmental events in plant cell growth and differentiation. Nucleation events play a key role in the dynamic and organization of the microtubule (Mt) cytoskeleton. Among many proteins involved in Mt nucleation, γ -tubulin has been identified as an essential component of the Mt organizing centers (MTOC). In protoplasts, somatic embryogenesis induction has been correlated with remodeling of Mt cytoskeleton. We have investigated the specific developmental expression of γ -tubulin in Helianthus annuus . Two γ -tubulin isoforms have been detected by immunoblotting, with bands at 52 and 58 kDa. The larger γ -tubulin (58 kDa) is present in all the sunflower tissues tested and is associated with the nucleus. The smaller γ -tubulin (52 kDa), differing from the former at the carboxy-terminal end, is only present in meristematic and dedifferentiated cells and is not bound to the nucleus. This first demonstration of the presence of two γ -tubulins in plant cells is discussed in terms of distinct roles in the nucleation and organization of Mts.     

Specifying positional information in the embryo: looking beyond morphogens

Concentration gradients of small diffusible molecules called morphogens are key regulators of development, specifying position during pattern formation in the embryo. It is now becoming clear that additional or alternative mechanisms involving interactions among cells are also crucial for positional specification.