Comparative sequence analysis of primate subtelomeres originating from a chromosome fission event.

Subtelomeres are concentrations of interchromosomal segmental duplications capped by telomeric repeats at the ends of chromosomes. The nature of the segments shared by different sets of human subtelomeres reflects their high rate of recent interchromosomal exchange. Here, we characterize the rearrangements incurred by the 15q subtelomere after it arose from a chromosome fission event in the common ancestor of great apes. We used FISH, sequencing of genomic clones, and PCR to map the breakpoint of this fission and track the fate of flanking sequence in human, chimpanzee, gorilla, orangutan, and macaque genomes. The ancestral locus, a cluster of olfactory receptor (OR) genes, lies internally on macaque chromosome 7. Sequence originating from this fission site is split between the terminus of 15q and the pericentromere of 14q in the great apes. Numerous structural rearrangements, including interstitial deletions and transfers of material to or from other subtelomeres, occurred subsequent to the fission, such that each species has a unique 15q structure and unique collection of ORs derived from the fission locus. The most striking rearrangement involved transfer of at least 200 kb from the fission-site region to the end of chromosome 4q, where much still resides in chimpanzee and gorilla, but not in human. This gross structural difference places the subtelomeric defect underlying facioscapulohumeral muscular dystrophy (FSHD) much closer to the telomere in human 4q than in the hybrid 4q-15q subtelomere of chimpanzee.

Extensive copy-number variation of the human olfactory receptor gene family.

As much as a quarter of the human genome has been reported to vary in copy number between individuals, including regions containing about half of the members of the olfactory receptor (OR) gene family. We have undertaken a detailed study of copy-number variation of ORs to elucidate the selective and mechanistic forces acting on this gene family and the true impact of copy-number variation on human OR repertoires. We argue that the properties of copy-number variants (CNVs) and other sets of large genomic regions violate the assumptions of statistical methods that are commonly used in the assessment of gene enrichment. Using more appropriate methods, we provide evidence that OR enrichment in CNVs is not due to positive selection but is because of OR preponderance in segmentally duplicated regions, which are known to be frequently copy-number variable, and because purifying selection against CNVs is lower in OR-containing regions than in regions containing essential genes. We also combine multiplex ligation-dependent probe amplification (MLPA) and PCR to assay the copy numbers of 37 candidate CNV ORs in a panel of approximately 50 human individuals. We confirm copy-number variation of 18 ORs but find no variation in this human-diversity panel for 16 other ORs, highlighting the caveat that reported intervals often overrepresent true CNVs. The copy-number variation we describe is likely to underpin significant variation in olfactory abilities among human individuals. Finally, we show that both homology-based and homology-independent processes have played a recent role in remodeling the OR family.

Investigating molecular recognition and biological function at interfaces using piscidins, antimicrobial peptides from fish.

We studied amidated and non-amidated piscidins 1 and 3, amphipathic cationic antimicrobial peptides from fish, to characterize functional and structural similarities and differences between these peptides and better understand the structural motifs involved in biological activity and functional diversity among amidated and non-amidated isoforms. Antimicrobial and hemolytic assays were carried out to assess their potency and toxicity, respectively. Site-specific high-resolution solid-state NMR orientational restraints were obtained from (15)N-labeled amidated and non-amidated piscidins 1 and 3 in the presence of hydrated oriented lipid bilayers. Solid-state NMR and circular dichroism results indicate that the peptides are alpha-helical and oriented parallel to the membrane surface. This orientation was expected since peptide-lipid interactions are enhanced at the water-bilayer interface for amphipathic cationic antimicrobial peptides. (15)N solid-state NMR performed on oriented samples demonstrate that piscidin experiences fast, large amplitude backbone motions around an axis parallel to the bilayer normal. Under the conditions tested here, piscidin 1 was confirmed to be more antimicrobially potent than piscidin 3 and antimicrobial activity was not affected by amidation. In light of functional and structural similarities between piscidins 1 and 3, we propose that their topology and fast dynamics are related to their mechanism of action.