Abstract. The boundary between the North American andCaribbean plates is characterized primarily by left-lateral motion along predominantly east-west striking faults. Seismicity and marine geophysical survey data are consistent with at least two, and possibly three, microplates in the diffuse boundary zone in the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) the Puerto Rico-northern Virgin Islands (PRVI). We discuss results fi'om GPS geodetic measurements acquired since 1994 to test the microplate hypothesis, define PRVI translation and rotation within the boundary

S U M M A R YWe present a re-evaluation of the seismological properties of the Aleutian 'tsunami earthquake' of 1946 April 1, characterized by a deceptively low conventional magnitude (7.4) in view of its catastrophic tsunami, both in the near and far fields. Relocation of 40 aftershocks show that the fault zone extends a minimum of 181 km along the Aleutian trench, in a geometry requiring a bilateral rupture from the original nucleation at the epicentre. Their spatial and temporal distribution are typical of the aftershock patterns of a large earthquake, and rule out the model of a landslide source exclusive of a dislocation. The analysis of the spectra of mantle waves favours the model of a large seismic source, with a static moment of 8.5 × 10 28 dyn-cm, making the event one of the ten largest earthquakes ever recorded (hence the destructive tsunami in the far field), and of a slow bilateral rupture, at an average velocity of only 1.12 km s −1 , hence the destructive interference in all azimuths for all but the longest mantle waves. The exceptionally slow character of the earthquake is confirmed by a deficiency in radiated seismic energy expressed by the lowest value measured to date of the energy-tomoment ratio. The earthquake appears as an end member in the family of 'tsunami earthquakes', resulting from the combination of anomalous, but not unprecedented, parameters, such as low stress drop and rupture velocity.

The type II secretion system (T2SS) is a multi-protein envelope-spanning assembly that 25 translocates a wide range of virulence factors, enzymes and effectors through the outer 26 membrane (OM) of many Gram-negative bacteria. Here, using electron cryotomography and 27 subtomogram averaging methods, we present the first in situ structure of an intact T2SS, imaged 28 within the human pathogen Legionella pneumophila. Although the T2SS has only limited 29 sequence and component homology with the evolutionarily-related Type IV pilus (T4P) system, 30 we show that their overall architectures are remarkably similar. Despite similarities, there are 31 also differences, including for instance that the T2SS-ATPase complex is usually present but 32 disengaged from the inner membrane, the T2SS has a much longer periplasmic vestibule, and it 33 has a short-lived flexible pseudopilus. Placing atomic models of the components into our ECT 34 map produced a complete architectural model of the intact T2SS that provides new insights into 35 the structure and function of its components, its position within the cell envelope, and the 36 interactions between its different subcomplexes. Overall, these structural results strongly support 37 the piston model for substrate extrusion. 38 39 40 All rights reserved. No reuse allowed without permission.

Stenotrophomonas maltophilia is an emerging opportunistic and nosocomial pathogen. S. maltophilia is also a risk factor for lung exacerbations in cystic fibrosis patients. S. maltophilia attaches to various mammalian cells, and we recently documented that the bacterium encodes a type II secretion system which triggers detachment-induced apoptosis in lung epithelial cells. We have now confirmed that S. maltophilia also encodes a type IVA secretion system (VirB/VirD4 [VirB/D4] T4SS) that is highly conserved among S. maltophilia strains and, looking beyond the Stenotrophomonas genus, is most similar to the T4SS of Xanthomonas. To define the role(s) of this T4SS, we constructed a mutant of strain K279a that is devoid of secretion activity due to loss of the VirB10 component. The mutant induced a higher level of apoptosis upon infection of human lung epithelial cells, indicating that a T4SS effector(s) has antiapoptotic activity. However, when we infected human macrophages, the mutant triggered a lower level of apoptosis, implying that the T4SS also elaborates a proapoptotic factor(s). Moreover, when we cocultured K279a with strains of Pseudomonas aeruginosa, the T4SS promoted the growth of S. maltophilia and reduced the numbers of heterologous bacteria, signaling that another effector(s) has antibacterial activity. In all cases, the effect of the T4SS required S. maltophilia contact with its target. Thus, S. maltophilia VirB/D4 T4SS appears to secrete multiple effectors capable of modulating death pathways. That a T4SS can have anti- and prokilling effects on different targets, including both human and bacterial cells, has, to our knowledge, not been seen before.

The high diversity of soil bacteria is attributed to the spatial complexity of soil systems, where habitat heterogeneity promotes niche partitioning among bacterial taxa. This premise remains challenging to test, however, as it requires quantifying the traits of closely related soil bacteria and relating these traits to bacterial abundances and geographic distributions. Here, we sought to investigate whether the widespread soil taxon Curtobacterium consists of multiple coexisting ecotypes with differential geographic distributions. We isolated Curtobacterium strains from six sites along a climate gradient and assayed four functional traits that may contribute to niche partitioning in leaf litter, the top layer of soil. Our results revealed that cultured isolates separated into fine-scale genetic clusters that reflected distinct suites of phenotypic traits, denoting the existence of multiple ecotypes. We then quantified the distribution of Curtobacterium by analysing metagenomic data collected across the gradient over 18 months. Six abundant ecotypes were observed with differential abundances along the gradient, suggesting fine-scale niche partitioning. However, we could not clearly explain observed geographic distributions of ecotypes by relating their traits to environmental variables. Thus, while we can resolve soil bacterial ecotypes, the traits delineating their distinct niches in the environment remain unclear.