Effects of Bulkiness and Hydrophobicity of an Aliphatic Amino Acid in the Recognition Helix of the GAGA Zinc Finger on the Stability of the Hydrophobic Core and DNA Binding Affinity

Natural Selection in Virulence Genes of Francisella tularensis

A fundamental tenet of evolution is that alleles that are under negative selection are often deleterious and confer no evolutionary advantage. Negatively selected alleles are removed from the gene pool and are eventually extinguished from the population. Conversely, alleles under positive selection do confer an evolutionary advantage and lead to an increase in the overall fitness of the organism. These alleles increase in frequency until they eventually become fixed in the population. Francisella tularensis is a zoonotic pathogen and a potential biothreat agent. The most virulent type of F. tularensis, Type A, is distributed across North America with Type A.I occurring mainly in the east and Type A.II appearing mainly in the west. F. tularensis is thought to be a genome in decay (losing genes) because of the relatively large number of pseudogenes present in its genome. We hypothesized that the observed frequency of gene loss/pseudogenes may be an artifact of evolution in response to a changing environment, and that genes involved in virulence should be under strong positive selection. To test this hypothesis, we sequenced and compared whole genomes of Type A.I and A.II isolates. We analyzed a subset of virulence and housekeeping genes from several F. tularensis subspecies genomes to ascertain the presence and extent of positive selection. Eleven previously identified virulence genes were screened for positive selection along with 10 housekeeping genes. Analyses of selection yielded one housekeeping gene and 7 virulence genes which showed significant evidence of positive selection at loci implicated in cell surface structures and membrane proteins, metabolism and biosynthesis, transcription, translation and cell separation, and substrate binding and transport. Our results suggest that while the loss of functional genes through disuse could be accelerated by negative selection, the genome decay in Francisella could also be the byproduct of adaptive evolution driven by complex interactions between host, pathogen, and thier environment, as evidenced by several of its virulence genes which are undergoing strong, positive selection.

Zn(II) binding and DNA binding properties of ligand-substituted CXHH-type zinc finger proteins

CCHH-type zinc fingers are among the most common DNA binding motifs found in eukaryotes. In a previous report, we substituted the second ligand cysteine residue with aspartic acid, producing a Zn(II)-responsive transcription factor; this indicates that a ligand substitution is a possible design target of an engineered zinc finger peptide. Despite the importance of Zn(II) binding with respect to the folding and DNA binding properties of a zinc finger peptide, no study about the effects of ligand substitution on both Zn(II) binding and DNA binding properties has been reported. Here, we substituted a conserved cysteine (C) with other zinc-coordinated amino acid residues, histidine (H), aspartic acid (D), and glutamic acid (E), to create CXHH-type zinc finger peptides (X = C, H, D, and E). The Zn(II)-dependent conformational change was observed in all peptides; however, the Zn(II) binding affinity and metal coordination geometry of the peptides were different. Gel mobility shift assays showed that the Zn(II)-bound forms of the ligand-substituted derivatives retain DNA binding ability, while the DNA binding affinity decreased in the following manner: CCHH > CDHH > CEHH ≫ CHHH. The DNA binding sequence preferences of the ligand-substituted derivatives were similar to that of the wild type in the context of the full three-finger DNA-binding domain of transcription factor Zif268. These results indicate that artificial zinc finger proteins with various DNA binding affinities that respond to a diverse range of Zn(II) concentrations can be designed by substituting the Zn(II) ligand.