F2 Structures Research Articles

Structural bases for F plasmid conjugation and F pilus biogenesis in Escherichia coli

Bacterial conjugation systems are members of the large type IV secretion system (T4SS) superfamily. Conjugative transfer of F plasmids residing in the Enterobacteriaceae was first reported in the 1940s, yet the architecture of F plasmid-encoded transfer channel and its physical relationship with the F pilus remain unknown. We visualized F-encoded structures in the native bacterial cell envelope by in situ cryoelectron tomography (CryoET). Remarkably, F plasmids encode four distinct structures, not just the translocation channel or channel-pilus complex predicted by prevailing models. The F1 structure is composed of distinct outer and inner membrane complexes and a connecting cylinder that together house the envelope-spanning translocation channel. The F2 structure is essentially the F1 complex with the F pilus attached at the outer membrane (OM). Remarkably, the F3 structure consists of the F pilus attached to a thin, cell envelope-spanning stalk, whereas the F4 structure consists of the pilus docked to the OM without an associated periplasmic density. The traffic ATPase TraC is configured as a hexamer of dimers at the cytoplasmic faces of the F1 and F2 structures, where it respectively regulates substrate transfer and F pilus biogenesis. Together, our findings present architectural renderings of the DNA conjugation or "mating" channel, the channel-pilus connection, and unprecedented pilus basal structures. These structural snapshots support a model for biogenesis of the F transfer system and allow for detailed comparisons with other structurally characterized T4SSs.

Preliminary investigation of a major high-strain zone in the Caledonian Highlands, southern New Brunswick

A major ductile high-strain zone up to 5 km in width can be traced for at least 70 km diagonally across the Avalonian Caledonia terrane of southern New Brunswick. A study of the northeastern part of this zone from the Prosser Mountain area in the northwest to the Point Wolfe River area west of Fundy National Park shows that both the ca. 630-620 Ma Broad River Group and associated plutons and the 560-550 Ma Coldbrook Group contain similar structural elements, related to a largely shared deformational history. Some of this history is apparent also in the 560–550 Ma plutonic rocks. A pervasive foliation (S1) lies parallel to bedding (S0), and although evidently composite (S0-1) in the Broad River Group, this fabric is very heterogeneous in the younger Coldbrook Group, where low strain enclaves are widespread. No folds have been seen of an F1 generation, and no reversals of facing or vergence are apparent. A mineral lineation (L1m) is locally prominent. The plutonic rocks have early fabrics, including a foliation (S1) producing augen-gneiss with a prominent L-tectonite (L1m). S1 also includes a schistosity associated with the growth of white mica and breakdown of feldspar. Geometry suggests that S1 in the granites is related to S0-1 in the supracrustal rocks, and L1m in both units shares a common orientation. S1 and S0-1 are crenulated by a strong second cleavage (S2) axial planar to folds (F2), the large-scale expression of which is an asymmetric synform containing a belt of Coldbrook Group rocks. Kinematic indicators suggest an overall top-to-the-SE motion along thrusts that stack units of Broad River Group, Coldbrook Group, and plutonic rocks. Fabric development in the plutonic rocks implies a history of exhumation beginning under hot, anhydrous conditions, followed by hydration during retrogression as plutonic rocks were tectonically emplaced into this crustal stack. The age of these tectonic events is not yet well constrained, but could be as young as Carboniferous.
 
 RÉSUMÉ
 
 Il est possible de retracer une importante zone de forte contrainte ductile ayant jusqu’à cinq kilomètres de largeur sur une distance de 70 kilomètres en diagonale à travers le terrane avalonien de Caledonia, dans le Sud du Nouveau-Brunswick. Une étude de la partie nord-est de cette zone à partir du secteur du mont Prosser, dans le nord-ouest, jusqu’au secteur de la rivière Pointe Wolfe, à l’ouest du parc national du Canada Fundy, révèle que le groupe d’environ 630 à 620 Ma de la rivière Broad, les plutons connexes et le groupe de 560 à 550 Ma de Coldbrook abritent des éléments structuraux similaires, apparentés à des déformations passées largement partagées. Une certaine partie de ce passé est également apparente dans les roches plutoniques de 560 à 550 Ma. Une foliation intense (S1) se manifeste parallèlement à la stratification (S0) et, même si cette fabrique est nettement composite (S0-1) dans le groupe de la rivière Broad, elle est très hétérogène dans le groupe plus récent de Coldbrook, où les enclaves de faible contrainte sont répandues. On n’a observé aucun pli d’une production F1 et aucune inversion du regard ni de la vergence n’est apparente. Une linéation minérale (L1m), définie par des agrégats de biotite, est localement bien visible. Les roches plutoniques possèdent des fabriques qui se sont constituées pendant et peu après la cristallisation, notamment une foliation (S1) produisant du gneiss oeillé avec L-tectonite (L1m) en évidence. S1 à l’intérieur des roches plutoniques comporte en outre une schistosité associée à la croissance de mica blanc et à la décomposition de feldspath. La géométrie permet de supposer que S1 dans les granites est apparentée à S0-1 dans les roches supracrustales et que la linéation minérale (L1m) dans les deux unités partage une orientation commune. S1 et S0-1 sont crénelés par une seconde schistosité (S2) prononcée, de plan axial par rapport aux plis (F2), dont l’expression à grande échelle est une synforme asymétrique renfermant une ceinture de roches du groupe de Coldbrook. Les indicateurs cinématiques des structures F2 permettent de supposer un mouvement général du sommet vers le sud-est le long des chevauchements qui empilent les unités du groupe de la rivière Broad, du groupe de Coldbrook et des roches plutoniques. Le développement de la fabrique dans les roches plutoniques plus âgées suppose une exhumation passée ayant commencé dans des conditions très chaudes et anhydres pendant et peu après la cristallisation, vers 620 Ma, suivie par une hydratation pendant la rétrogression au moment où les roches plutoniques se sont tectoniquement mises en place à l’intérieur de cet éperon crustal. L’âge des événements tectoniques tardifs n’est pas encore bien circonscrit, mais ils pourraient remonter au Carbonifère. [Traduit par la redaction]

Structure and stratigraphy of the south flank of the Kisseynew Domain in the Trans-Hudson Orogen, Manitoba: implications for 1.845-1.77 Ga collision tectonics

On the south flank of the Kisseynew Domain, orthogneisses derived from 1.92-1.85 Ga volcano-plutonic rocks are overlain by paragneisses (Burntwood and Missi groups) derived from 1.855-1.84 Ga marine turbidite and 1.845-1.83 Ga terrestrial clastic and volcanic rocks. The sediments in these groups are interpreted as having been shed into the Kisseynew paleobasin from an active margin bordering the Flin Flon Belt. The sedimentation apparently followed early microcontinental collision and accompanied the last arc magmatism in the Trans-Hudson Orogen. The sedimentary rocks and their basement were deformed into a complexly refolded stack of large recumbent folds. Premetamorphic F1 structures represent a fold and thrust system initiated during the sedimentation. These structures are interpreted as transported toward the Kisseynew Domain in the northeast and the hinterland in the southwest. F2 structures (~1.82 Ga) comprise westerly transported nappes. During 1.82-1.80 Ga high-grade metamorphism, the early structures were overturned, amplified, and refolded. Basement-cored culminations and sheet-like synforms of paragneiss were horizontally attenuated and transported south and southwest. North- and northeast-trending F4 folds and F5 faults formed after 1.79 Ga. The whole cycle of deformation is related to stages of continental collision between the internal (juvenile) zone of the Trans-Hudson Orogen and the three surrounding Archean cratons (Sask, Superior, and Hearne). The F4 upright folds and steep F5 faults are interpreted as the record of intracontinental transpression, strongly controlled by the Superior Craton boundary.

Non-trough <i>fo</i>F2 enhancements at near-equatorial dip latitudes

Abstract. Fine resolution series from three equatorial ionosondes of the IEEY network in West Africa have revealed small-scale daytime peak F2 structures, superposed on the slowly varying minimum or "trough" distribution in the ±5° magnetic latitude zone. We report this new morphology, concentrating on foF2 enhancements of two types: near-equatorial crests (which travel either northwards or southwards) and magnetic field-aligned domes, whose onsets last only tens of minutes. Both types are observed to start at mid-morning or early afternoon hours. We relate their occurrence with the available variations of Vz=E × B upward drift which feeds the equatorial plasma fountain. We suggest the foF2 enhancements to be triggered by brief slow-downs of the Vz velocity near F2 peak altitude in our West African sector. Their short latitude extent differentiates them from the larger-scale tropical crest system. Further analysis of these features should lead to weather-like models of the low latitude ionosphere variations, where unstable local coupling between processes seems to be the trigger.Key words. Ionosphere (Equatorial ionosphere · Ionosphere-atmosphere interaction · Plasma temperature and density)

Stratigraphy and ductile structure of the Presidential Range, New Hampshire: Tectonic implications for the Acadian orogeny

The Presidential Range of New Hampshire with its unique high relief contains exceptionally well exposed rocks of the Acadian orogenic hinterland. These rocks are within the Central Maine terrane, a belt of complexly metamorphosed and deformed Silurian and Devonian metasedimentary rocks. The Presidential Range lies between regions to the south dominated by refolded nappes and synkinematic high-grade regional metamorphism and regions to the northeast dominated by overlapping multiple deformation and contact metamorphism produced by late- and post-Acadian plutons. To better understand these complex tectonic variations and, in particular, to clearly and accurately determine the effects of the Devonian Acadian orogeny in a well-exposed region, the bedrock geology in the alpine zone of the Presidential Range was mapped in great detail. The results of the stratigraphic and ductile structural analyses are presented here. Five metasedimentary formations have been recognized and constitute a conformable stratigraphy, which, from oldest to youngest, are the Silurian (?) Rangeley, Perry Mountain, Smalls Falls, and Madrid Formations, and the Devonian (?) Littleton Formation. The Littleton Formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists and quartzites and other lithologic peculiarities. The Madrid, Smalls Falls, and Perry Mountains Formations are extremely thin, consistent with their deposition near the Silurian tectonic hinge. The Rangeley Formation is interpreted as an olistostromal melange and has been subdivided into three different members and six submembers on the basis of lithologic variations in the gneiss and subordinate units. The Presidential Range preserves three phases of ductile folding (F1, F2, and F3) and a single phase of thrust faulting (T1). Well-constrained southeast-, east- or northeast-facing directions and consistent asymmetry of F1 nappe limbs suggest that nappe vergence was easterly. Severe refolding of some nappes resulted during F2 collapse of the nappe pile. The west-vergent (?) Greenough Spring thrust fault truncated F1 and F2 structures and juxtaposed nonmigmatized and migmatized rocks. F3 east-vergent folding dominates the alpine zone and represents the final tightening of Acadian structures, which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets. The structural and sedimentological evidence supports the existence of a subduction system that was active beginning in the early Silurian on the east flank of the Bronson Hill Anticlinorium with a west-dipping subducting slab geometry. This system persisted throughout the Acadian.

Crystalline defects in heteroepitaxial GaAs/(Ca,Sr)F2 grown by molecular-beam epitaxy

The crystallinity of GaAs/(Ca,Sr)F2 structures grown by molecular-beam epitaxy on (100) and (111) B-oriented GaAs substrates is studied by transmission electron microscopy. The main defects found in the GaAs heteroepitaxial layers were microtwins and dislocations. The geometry and the origin of these defects are discussed in terms of growth and stress relaxation mechanisms, and placed within the framework of published results on similar heteroepitaxial systems. The results obtained on (111)-oriented layers suggest the preeminent role played by stress at growth temperature.

Time-resolved synchrotron X-ray study of chain-folded crystallization of long paraffins

The examination of the crystallization of strictly uniform ultra-long n-alkanes was resumed with the view of exploring the onset of chain-folding, as laid out previously in ref. 1. The present study, largely on C 246H 494, centred on the initial stages of crystallization in the melt, registered in situ by time-resolved small-angle X-ray scattering using a synchrotron X-ray source. The salient new feature was the identification of transient initial fold lengths which were non-integer fractions (NIF) of the chain length. This NIF structure transforms subsequently into forms with integer fraction (IF) fold lengths. In the present study the latter have the extended chain (E) and once-folded (F2) configurations, while NIF has a fold length between the two. The NIF → IF transformation occurs either by lamellar thickening or thinning, or by both. It was found that the NIF state had a more disordered layer surface as compared to the final E and F2 structures, the latter being the states on which the conclusions in ref. 1 had been drawn, which accordingly should apply to the transformed material. Implications of these and several other findings for some central issues in polymer crystallization are briefly discussed. The existence of an initial NIF phase focusses attention to the importance of the fastest kinetic pathway as the determining factor for chain-folded crystal growth with particular attention to the initial chain deposition probability, a line made accessible by the present alkanes.

Petrology, structure and age of the Rough Point sill, Donegal

AbstractThe Rough Point sill, which is one of the thickest of a large number of sills intruded into the Appin Group metasediments of the Dalradian sequence in north Donegal, contains a number of internal features including mafic:felsic banding, pegmatite bands and significant variations in grain size. There are also appreciable variations in mineralogy, particularly the proportions of plagioclase and amphibole, and in the major and trace element chemistry. Together these features indicate that the sill is, like the metasediments which it intrudes, inverted, and is therefore older than the Caledonian F2 structures which caused the inversion. Disruption of the sediments at the sill contact is also earlier than the F2 event. The timing of the basic magmatic episode together with the geochemistry of the sill invite comparison with the metadolerites of the southwest Highlands of Scotland.

The structural geometry and geological history of Archean rocks at Kenora, north-western Ontario—a proposed type area for the Kenoran Orogeny

The Kenora area, at the south margin of the English River Subprovince, consists of polydeformed gneisses intruded by granitoid plutons, some of which are also deformed. The gneisses are mainly tonalites of igneous origin, but contain remnants of an earlier, dominantly mafic, supracrustal sequence and severely deformed pegmatites.Within the gneissosity are intrafolial F1 folds subsequently refolded by tight-to-open F2 folds. Major F2 folds can be delineated from the geometry and distribution of minor F2 structures. During deformation the area was subjected to upper amphibolite facies metamorphism (650–750 °C, 4–7 kbar (400–700 MPa)) with K-feldspar megacrysts and garnet developed in the deeper levels. Major F2 folds are refolded in a rim synform (F3), which developed synchronously with the emplacement of the Dalles granodiorite, an event that also created a plunge culmination uplifting part of the deeper K-feldspar megacryst – garnet structural level. Cumulative apparent dextral displacement of 6 km in two periods of movement, related partly to the emplacement of the Dalles granodiorite, is suggested for a major fault 1.5 km north of, and parallel to, the English River – Wabigoon Subprovince interface. Open, upright F4 folds caused buckling of dioritic dikes and pegmatites. Later deformation involved minor faulting and joint development.Rb–Sr isotopic studies indicate that this geological history took place within about 150 Ma with most activity over by 2600 Ma. It is proposed that this should be selected as the type area for the Kenoran Orogeny.

Superposed folding and metamorphism in the Seve nappe around Åreskutan in the Swedish Caledonides

Abstract The earliest recognisable folds (F1) in the Seve nappe in the Åre area in the central Swedish Caledonides include a phase of co-axial refolding (F1A and F1B). These isoclinal, tight reclined or recumbent folds are parallel to the principal mineral lineation and mylonitic striation. There is a set of tight, reclined folds (F2) which refold the mineral lineation and F1 hinges at large angles. The open F3 folds refold the F2 hinges at small angles. In addition to the above structures, there is also a set of open, upright folds (F1) with axes parallel to the mineral lineation (and to F1) which is younger than F1. The time-relation of these F1 folds to the F2 folds is not clear. The field evidence indicates that F1B and later folds have developed during different stages of thrusting. The period of migmatization and high-grade metamorphism within the nappe has overlapped with the period of development of the F1 and F2 structures.

Metamorphism, structure and stratigraphy in the Southern Park Ranges, British Columbia

The Southern Park Ranges lie immediately east of the Rocky Mountain Trench, near the Big Bend of the Columbia River, B.C. They are composed of quartzites, pelites and carbonates in an uninterrupted sequence from the Proterozoic Windermere Supergroup, through the Lower Cambrian Gog Group, to the Middle Cambrian Chancellor Group.Two phases of folding and metamorphism are recognized. The first phase of folding (F1) occurred before and during the first phase of metamorphism (M1), whereas the second phase of folding (F2) was associated with a second (very minor) phase of metamorphism (M2). Both F1 and F2 structures are apparently associated with the rise of the Porcupine Creek Anticlinorium, a major fold complex trending northwest–southeast across the eastern part of the study area. Associated with F2 folds are several west-dipping thrust faults, some of which terminate upwards in the cores of F2 anticlines. All structures are cut by late-stage, west-dipping normal faults.The first phase of metamorphism (M1) resulted in a Barrovian sequence of isograds such that chlorite zone rocks occur in the east and sillimanite zone rocks occur in the west. Phase equilibria studies suggest that the metamorphic temperatures in the area ranged from less than 480 °C (east) to 570 °C (west) at a pressure of approximately 5 kbar (5 × 105 kPa).The M1 isograds in the Park Ranges outline a thermal antiform whose western limb is truncated by the Purcell Fault (a major Cordilleran structure) on the western slopes of the study area.A comparison of main metamorphic phases between the Park Ranges and the Selkirk and Monashee Mountains across the Purcell Fault, suggests that the western (Selkirk–Monashee) terrane underwent temperatures approximately 40 °C higher than the study area (for equivalent metamorphic zones) and a pressure between 2 and 3 kbar (2 × 105 and 3 × 105 kPa) higher than the study area. This pressure difference suggests that there has been a minimum of 7 km post-metamorphic vertical motion on the Purcell Fault. A difference in structural style across the Purcell Fault, due to a higher ductility in the western terrane, is interpreted as a result of the difference in depth during the deformation of the two terranes.

Age and style of mesostructures in north-central Gaspé, Quebec

Three main geological units are distinguished in the Mount McGerrigle area of north-central Gaspé, each different in lithology, structural style, and age: a Cambro-Ordovician, complexly deformed assemblage of eugeosynclinal rocks is discordantly intruded by a composite pluton, and is unconformably overlain by mildly deformed Silurian–Devonian platform deposits. Three phases of deformation (F1, F2, F3) have affected the Quebec and Schickshock Groups of Cambro-Ordovidian age. The F1, deformation was accompanied by lowgrade regional metamorphism resulting in a penetrative planar fabric. The F2 deformation deformed pre-existing structures and produced the prevalent northeasterly trending and shallowly plunging folds. The third phase was least penetrative, resulting in broad and upright east-northeasterly striking F3 folds that modify the orientation of F2 structures. The forcefully injected Mount McGerrigle pluton, a late intrusion of the Acadian orogeny, truncates and deforms the F1 and F2 fabric elements of the Cambro-Ordovician rock. Multiphase deformation thus preceded the Acadian orogeny in the area. Silurian–Devonian rocks, on the other hand, have been affected only by the F3 deformation during the Acadian orogeny.

Two high grade schistosities at broken hill and their relation to major and minor structures

Abstract The regional schistosity at Broken Hill, previously regarded as a single fabric element, has been separated into three overprinting elements—one of high meta‐morphic grade (S1), one of variable grade (S2), and one consistently retrograde (S3). S1 and S2 make up by far the majority of occurrences mapped to date. A new descriptive terminology is introduced to take account of this complexity. Extensive metamorphic crystallization has taken place during the first two periods of schistosity formation (F1, F2), and the distribution of metamorphic assemblages is thus partly controlled by the extent to which F1 assemblages have been modified by F2, and the variable grade of F2. A regional retrograde schistosity can locally be shown to be S2, although it is sometimes superposed on F2 structures and is then classified as S3. A number of major fold‐hinges have been mapped in the region, primarily using the extensive mesoscopic data available from one rock type—a layered sillimanite gneiss. Almost without ex...

Structural Studies on the Uranium Deposit of the Fay Mine, Eldorado, Northwest Saskatchewan

The Fay Mine uranium deposit is composed of veins parallel to the foliation of the paragneisses and paraschists of the Fay Mine Complex, which in turn overlies the Donaldson Lake Gneiss and Foot Bay Gneiss, within the Tazin Group of the Beaverlodge District, northern Saskatchewan. A computer oriented study of 5500 strike-dip measurements of planar features within the Fay Mine reveals that the lowest member of the metamorphic sequence, the Foot Bay Gneiss, is unique in that it exhibits Kenoran (?) structures associated with F1, NW-SE or N-S trending folds. Subsequent to the Kenoran (?) folding, an Aphebian (?) sequence of sediments and volcanics, now represented by the Donaldson Lake Gneiss and the Fay Mine Complex, was deposited. NE-SW trending, Hudsonian, F2 folds then developed and extensive cataclasis was effected. The F2 structures also affected the uppermost, taphrogeosynclinal Martin Formation sediments and volcanics. The major faults of the district are thought to have been initiated either as F2-shear joints or F2-tension fractures and have undergone multiple movement over a long period of time. The pitchblende ± quartz ± carbonates ± hematite ± chlorite veins, classified as 'mobilized stratabound deposits', occur parallel to S2 foliation planes in the crests, troughs, or limbs of F2 folds, wherever minimal plunge is observed. Such deposits may have been generated during regional metamorphism by mobilization of uranium-rich fractions from the originally uraniferous Aphebian precursors of the Fay Mine Complex. Later movement of such major structures as the St. Louis Fault has apparently caused only local deformation and brecciation of the uraniferous veins and has caused no more than limited recrystallization.

The structures of the Ardsheal Peninsula, Argyll, their age and regional significance

AbstractThe lithologies and structures of the Dalradian rocks of the Ardsheal Peninsula, Argyllshire are described. The structures associated with the main synformal fold (part of the Appin Syncline) are shown to be F1 in age. F2 structures cross the Syncline and show progressive development towards the east. The Syncline is correlated and compared with other major F1 folds in the region. This interpretation is contrasted with other interpretations of the Syncline as a third‐phase structure which can be correlated with late‐phase folds in the Dalradian.