Gene conversion and natural selection in the evolution of X-linked color vision genes in higher primates

During higher primate evolution, gene conversion seems to have occurred often between the red and green photo-pigment genes, which are tandemly linked on the X chromosome. To understand this phenomenon better, intron 4 sequences of the red and green pigment genes of a male human (an Asian Indian), a male chimpanzee, and a male baboon were amplified by PCR and sequenced. The data show that the intron 4 sequences between the two genes have been strongly or completely homogenized in the three species studied. Apparently recent gene conversion events have occurred in introns 4 of the red and green pigment genes in humans and chimpanzees. Two or more conversion events may have occurred at different times in introns 4 of the two pigment genes in baboons. The divergence between the two genes is significantly lower in intron 4 than in exons 4 and 5 in each species, contrary to the usual situation that introns evolve faster than exons. It is most likely that strong natural selection for maintaining the distinct functions of exons 4 and 5 of the red and green pigment genes has acted against sequence homogenization of these exons.      

Solution structure of RNase P RNA

The ribonucleoprotein enzyme ribonuclease P (RNase P) processes tRNAs by cleavage of precursor-tRNAs. RNase P is a ribozyme: The RNA component catalyzes tRNA maturation in vitro without proteins. Remarkable features of RNase P include multiple turnovers in vivo and ability to process diverse substrates. Structures of the bacterial RNase P, including full-length RNAs and a ternary complex with substrate, have been determined by X-ray crystallography. However, crystal structures of free RNA are significantly different from the ternary complex, and the solution structure of the RNA is unknown. Here, we report solution structures of three phylogenetically distinct bacterial RNase P RNAs from Escherichia coli, Agrobacterium tumefaciens, and Bacillus stearothermophilus, determined using small angle X-ray scattering (SAXS) and selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis. A combination of homology modeling, normal mode analysis, and molecular dynamics was used to refine the structural models against the empirical data of these RNAs in solution under the high ionic strength required for catalytic activity.     

Combining H/D exchange mass spectroscopy and computational docking reveals extended DNA-binding surface on uracil-DNA glycosylase

X-ray crystallography provides excellent structural data on protein-DNA interfaces, but crystallographic complexes typically contain only small fragments of large DNA molecules. We present a new approach that can use longer DNA substrates and reveal new protein-DNA interactions even in extensively studied systems. Our approach combines rigid-body computational docking with hydrogen/deuterium exchange mass spectrometry (DXMS). DXMS identifies solvent-exposed protein surfaces; docking is used to create a 3-dimensional model of the protein-DNA interaction. We investigated the enzyme uracil-DNA glycosylase (UNG), which detects and cleaves uracil from DNA. UNG was incubated with a 30 bp DNA fragment containing a single uracil, giving the complex with the abasic DNA product. Compared with free UNG, the UNG-DNA complex showed increased solvent protection at the UNG active site and at two regions outside the active site: residues 210-220 and 251-264. Computational docking also identified these two DNA-binding surfaces, but neither shows DNA contact in UNG-DNA crystallographic structures. Our results can be explained by separation of the two DNA strands on one side of the active site. These non-sequence-specific DNA-binding surfaces may aid local uracil search, contribute to binding the abasic DNA product and help present the DNA product to APE-1, the next enzyme on the DNA-repair pathway.     

Estimating distribution of hidden objects with drones: from tennis balls to manatees

Unmanned aerial vehicles (UAV), or drones, have been used widely in military applications, but more recently civilian applications have emerged (e.g., wildlife population monitoring, traffic monitoring, law enforcement, oil and gas pipeline threat detection). UAV can have several advantages over manned aircraft for wildlife surveys, including reduced ecological footprint, increased safety, and the ability to collect high-resolution geo-referenced imagery that can document the presence of species without the use of a human observer. We illustrate how geo-referenced data collected with UAV technology in combination with recently developed statistical models can improve our ability to estimate the distribution of organisms. To demonstrate the efficacy of this methodology, we conducted an experiment in which tennis balls were used as surrogates of organisms to be surveyed. We used a UAV to collect images of an experimental field with a known number of tennis balls, each of which had a certain probability of being hidden. We then applied spatially explicit occupancy models to estimate the number of balls and created precise distribution maps. We conducted three consecutive surveys over the experimental field and estimated the total number of balls to be 328 (95%CI: 312, 348). The true number was 329 balls, but simple counts based on the UAV pictures would have led to a total maximum count of 284. The distribution of the balls in the field followed a simulated environmental gradient. We also were able to accurately estimate the relationship between the gradient and the distribution of balls. Our experiment demonstrates how this technology can be used to create precise distribution maps in which discrete regions of the study area are assigned a probability of presence of an object. Finally, we discuss the applicability and relevance of this experimental study to the case study of Florida manatee distribution at power plants.     

Comparison and functional implications of the 3D architectures of viral tRNA-like structures

RNA viruses co-opt the host cell's biological machinery, and their infection strategies often depend on specific structures in the viral genomic RNA. Examples are tRNA-like structures (TLSs), found at the 3′ end of certain plant viral RNAs, which can use the cell's aminoacyl tRNA-synthetases (AARSs) to drive addition of an amino acid to the 3′ end of the viral RNA. TLSs are multifunctional RNAs involved in processes such as viral replication, translation, and viral RNA stability; these functions depend on their fold. Experimental result-based structural models of TLSs have been published. In this study, we further examine these structures using a combination of biophysical and biochemical approaches to explore the three-dimensional (3D) architectures of TLSs from the turnip yellow mosaic virus (TYMV), tobacco mosaic virus (TMV), and brome mosaic virus (BMV). We find that despite similar function, these RNAs are biophysically diverse: the TYMV TLS adopts a characteristic tRNA-like L shape, the BMV TLS has a large compact globular domain with several helical extensions, and the TMV TLS aggregates in solution. Both the TYMV and BMV TLS RNAs adopt structures with tight backbone packing and also with dynamic structural elements, suggesting complexities and subtleties that cannot be explained by simple tRNA mimicry. These results confirm some aspects of existing models and also indicate how these models can be improved. The biophysical characteristics of these TLSs show how these multifunctional RNAs might regulate various viral processes, including negative strand synthesis, and also allow comparison with other structured RNAs.     

Comparative genomics of chondrichthyan Hoxa clusters

Background  

The chondrichthyan or cartilaginous fish (chimeras, sharks, skates and rays) occupy an important phylogenetic position as the sister group to all other jawed vertebrates and as an early lineage to diverge from the vertebrate lineage following two whole genome duplication events in vertebrate evolution. There have been few comparative genomic analyses incorporating data from chondrichthyan fish and none comparing genomic information from within the group. We have sequenced the complete Hoxa cluster of the Little Skate (Leucoraja erinacea) and compared to the published Hoxa cluster of the Horn Shark (Heterodontus francisci) and to available data from the Elephant Shark (Callorhinchus milii) genome project.     

Achieving fidelity in homologous recombination despite extreme complexity: informed decisions by molecular profiling

In this issue of Molecular Cell, Savir and Tlusty (2010) apply signal detection theory to show that homologous recombination machinery is optimally tuned to find homologous DNA sequences within an exceptionally high background of heterologous sequences.     

Sterols and fatty acids of aflatoxin and non-aflatoxin producing isolates of Aspergillus

The fatty acids and sterols present in 5 isolates of Aspergillus flavus and 3 isolates of A. parasiticus were determined; 2 isolates within each species were aflatoxin producers. The 4 major fatty acids were 16:0, 18:0, 18:1 and 18:2 with a trace of 15:0 in one isolate and traces of 17:0 in 3 other isolates. Cholesterol, ergosterol and 5, 7-ergostadienol were present in all isolates; the 5 isolates of A. flavus could be identified on the basis of retention times of minor sterols present. There was no correlation of total lipids, fatty acids or sterols with the production of aflatoxins. Water soluble complexes of sterols were not detected.     

Estimating the age of the common ancestor of a sample of DNA sequences

We present a simple Monte Carlo method for estimating the age of the most recent common ancestor (MRCA) of a sample of DNA sequences. We show that Templeton's (1993) estimator of the age of the MRCA based on the maximum number of nucleotide differences between two sequences in a sample is inaccurate, and we demonstrate the new method by reanalyzing a sample of DNA sequences from human Y chromosomes and a sample of human Alu sequences.