Insight into centromere-binding properties of ParB proteins: a secondary binding motif is essential for bacterial genome maintenance

ParB proteins are one of the three essential components of partition systems that actively segregate bacterial chromosomes and plasmids. In binding to centromere sequences, ParB assembles as nucleoprotein structures called partition complexes. These assemblies are the substrates for the partitioning process that ensures DNA molecules are segregated to both sides of the cell. We recently identified the sopC centromere nucleotides required for binding to the ParB homologue of plasmid F, SopB. This analysis also suggested a role in sopC binding for an arginine residue, R219, located outside the helix-turn-helix (HTH) DNA-binding motif previously shown to be the only determinant for sopC-specific binding. Here, we demonstrated that the R219 residue is critical for SopB binding to sopC during partition. Mutating R219 to alanine or lysine abolished partition by preventing partition complex assembly. Thus, specificity of SopB binding relies on two distinct motifs, an HTH and an arginine residue, which define a split DNA-binding domain larger than previously thought. Bioinformatic analysis over a broad range of chromosomal ParBs generalized our findings with the identification of a non-HTH positively charged residue essential for partition and centromere binding, present in a newly identified highly conserved motif. We propose that ParB proteins possess two DNA-binding motifs that form an extended centromere-binding domain, providing high specificity.     

Multiscale climate change impacts on plant diversity in the Atacama Desert

Comprehending ecological dynamics requires not only knowledge of modern communities but also detailed reconstructions of ecosystem history. Ancient DNA (aDNA) metabarcoding allows biodiversity responses to major climatic change to be explored at different spatial and temporal scales. We extracted aDNA preserved in fossil rodent middens to reconstruct late Quaternary vegetation dynamics in the hyperarid Atacama Desert. By comparing our paleo‐informed millennial record with contemporary observations of interannual variations in diversity, we show local plant communities behave differentially at different timescales. In the interannual (years to decades) time frame, only annual herbaceous expand and contract their distributional ranges (emerging from persistent seed banks) in response to precipitation, whereas perennials distribution appears to be extraordinarily resilient. In contrast, at longer timescales (thousands of years) many perennial species were displaced up to 1,000 m downslope during pluvial events. Given ongoing and future natural and anthropogenically induced climate change, our results not only provide baselines for vegetation in the Atacama Desert, but also help to inform how these and other high mountain plant communities may respond to fluctuations of climate in the future.     

Designing an integrated coastal orientation index: A cross-comparison of Mexican municipalities

To better understand the relation between coastal areas and their influence on social and economic conditions of municipalities, a set of indexes were developed to characterize coastal Mexico with an interdisciplinary approach. The composite of these bio-physical, social and economic indexes is referred to as an “integrated coastal orientation index”. Coastal characteristics should be taken into account in the coastal management policy formulation of a given region, and this index can be used to identify key areas for environmental protection or economic development at a national level. The results suggest that municipalities must manage their coastal jurisdictions, a large and diverse array of coastal space and ecosystems, medium to weak spatial demographics and settlement structures, infrastructure to improve quality of life, an overall low coastal population income, and various coastal activities. Spatial heterogeneity along the littoral zone in the coastal orientation led us to conduct a state analysis. We discuss these values and identify five highly coastal-orientated states in the extremes of the country. The development of this tool could help to pin-point different types of key areas within the regions studied and provide a tool to guide regional priorities. As an example, using a combination of extreme values of bio-physical, social and economic indexes was used to identify specific municipalities as candidates for environmental protection and others as areas where development can continue. Choices of indexes are discussed, and the relative importance given to each component of the coastal orientation (rather bio-physical, social and/or economic) will define the course of the coast's development and its sustainability.     

SAS‐6 coiled‐coil structure and interaction with SAS‐5 suggest a regulatory mechanism in C. elegans centriole assembly

The centriole is a conserved microtubule‐based organelle essential for both centrosome formation and cilium biogenesis. Five conserved proteins for centriole duplication have been identified. Two of them, SAS‐5 and SAS‐6, physically interact with each other and are codependent for their targeting to procentrioles. However, it remains unclear how these two proteins interact at the molecular level. Here, we demonstrate that the short SAS‐5 C‐terminal domain (residues 390–404) specifically binds to a narrow central region (residues 275–288) of the SAS‐6 coiled coil. This was supported by the crystal structure of the SAS‐6 coiled‐coil domain (CCD), which, together with mutagenesis studies, indicated that the association is mediated by synergistic hydrophobic and electrostatic interactions. The crystal structure also shows a periodic charge pattern along the SAS‐6 CCD, which gives rise to an anti‐parallel tetramer. Overall, our findings establish the molecular basis of the specific interaction between SAS‐5 and SAS‐6, and suggest that both proteins individually adopt an oligomeric conformation that is disrupted upon the formation of the hetero‐complex to facilitate the correct assembly of the nine‐fold symmetric centriole.     

Leaf Stripe and Stem Rot Caused by Burkholderia gladioli,a New Maize Disease in Mexico

A disease in maize plants, not previously observed, appeared in 2003–2004 in Cosoleacaque, Tlalixcoyan and Paso de Ovejas counties, in the state of Veracruz, Mexico. Initial symptoms on leaves were small white‐yellow watery spots, which coalesced into dry necrotic stripes 0.3 wide and up to 8 cm long. Reddening sometimes developed on these leaves. Stems developed a rot in the crown. The flag leaf became rot and necrotic at the base, rolled inwards and dried out. Necrosis developed at the base of the corn ears and their growth was inhibited. These symptoms were initially observed in Asgrow‐7573 commercial maize plantings. A bacterium characterized by white colonies, gram negative, aerobic, rod‐shape, opaque, round colonies with entire margins on casaaminoacid peptone and glucose media was consistently isolated from diseased maize plants. On King’s Medium B, the isolates produced yellow, non‐mucoid colonies, with the majority of the strains secreting a diffusible yellow pigment into the media. The bacterium identity was confirmed by PCR amplification and sequencing of 16S and 23S genes rDNA fragments. The bacterium was identified as Burkholderia gladioli. Its pathogenicity on maize plants in Mexico is a new record.     

Plant phylogenetic diversity of tropical mountaintop rocky grasslands: local and regional constraints

Plant Ecology - Mountains are interesting systems for studying patterns of diversity distribution and the role of environmental filters and competition on community assembly. According to the...     

Species formation and geographical range evolution in a genus of Central American cloud forest salamanders (Dendrotriton)

Aim Montane Central America offers an ideal system for testing geographical hypotheses of species diversification. We examined how the complex geological history of Nuclear Central America has shaped the diversification of a genus of cloud‐forest‐inhabiting salamanders (Dendrotriton). We applied parametric models of geographical range evolution to determine the predominant mode of species formation within the genus and to test existing hypotheses of geographical species formation in the region. Location Montane cloud forests of Nuclear Central America. Methods We estimated a species tree for Dendrotriton using a multi‐locus DNA sequence data set and several coalescent methods, and performed molecular dating for divergence events within the genus. We then applied the species‐tree estimate to a likelihood‐based time‐stratified model of geographical range evolution, based on current species distributions and available geological information for Central America. Results Species trees from all methods contain two groups, one corresponding to species from the Sierra de los Cuchumatanes and the other containing all remaining species. In most cases, species formation within the genus involved an even division of the geographical range of the ancestral species between descendant species. The ancestor of extant Dendrotriton species was estimated to have occurred in either the Sierra de los Cuchumatanes or the Sierra Madre de Chiapas, and both of these areas appear to have been important for diversification within the genus. The single species found in the Quaternary‐age Guatemalan volcanic cordillera dispersed to the volcanoes from an older highland area. Main conclusions Models of geographical range evolution, when combined with robust species‐tree estimates, provide insight into the historical biogeography of taxa not available from phylogenies or distributional data alone. Vicariant species formation, rather than peripatric or gradient speciation, appears to have been the dominant process of diversification, with most divergence events occurring within or between ancient highland areas. The apparent dispersal of Dendrotriton to the Quaternary‐age volcanoes raises the possibility that the rich salamander community there is composed of species that dispersed from geologically older areas. The Motagua Valley appears not to have been as important in vicariant species formation within Dendrotriton as it is within other groups.