Nucleotide sequences provide evidence of genetic exchange among distantly related lineages of Trypanosoma cruzi

Simple phylogenetic tests were applied to a large data set of nucleotide sequences from two nuclear genes and a region of the mitochondrial genome of Trypanosoma cruzi, the agent of Chagas' disease. Incongruent gene genealogies manifest genetic exchange among distantly related lineages of T. cruzi. Two widely distributed isoenzyme types of T. cruzi are hybrids, their genetic composition being the likely result of genetic exchange between two distantly related lineages. The data show that the reference strain for the T. cruzi genome project (CL Brener) is a hybrid. Well-supported gene genealogies show that mitochondrial and nuclear gene sequences from T. cruzi cluster, respectively, in three or four distinct clades that do not fully correspond to the two previously defined major lineages of T. cruzi. There is clear genetic differentiation among the major groups of sequences, but genetic diversity within each major group is low. We estimate that the major extant lineages of T. cruzi have diverged during the Miocene or early Pliocene (3–16 million years ago).

Library outreach: addressing Utah's “Digital Divide”

A “Digital Divide” in information and technological literacy exists in Utah between small hospitals and clinics in rural areas and the larger health care institutions in the major urban area of the state. The goals of the outreach program of the Spencer S. Eccles Health Sciences Library at the University of Utah address solutions to this disparity in partnership with the National Network of Libraries of Medicine—Midcontinental Region, the Utah Department of Health, and the Utah Area Health Education Centers. In a circuit-rider approach, an outreach librarian offers classes and demonstrations throughout the state that teach information-access skills to health professionals. Provision of traditional library services to unaffiliated health professionals is integrated into the library's daily workload as a component of the outreach program. The paper describes the history, methodology, administration, funding, impact, and results of the program.

Rad54 protein stimulates the postsynaptic phase of Rad51 protein-mediated DNA strand exchange

Rad54 and Rad51 are important proteins for the repair of double-stranded DNA breaks by homologous recombination in eukaryotes. As previously shown, Rad51 protein forms nucleoprotein filaments on single-stranded DNA, and Rad54 protein directly interacts with such filaments to enhance synapsis, the homologous pairing with a double-stranded DNA partner. Here we demonstrate that Saccharomyces cerevisiae Rad54 protein has an additional role in the postsynaptic phase of DNA strand exchange by stimulating heteroduplex DNA extension of established joint molecules in Rad51/Rpa-mediated DNA strand exchange. This function depended on the ATPase activity of Rad54 protein and on specific protein:protein interactions between the yeast Rad54 and Rad51 proteins.

DNA replication meets genetic exchange: Chromosomal damage and its repair by homologous recombination

Proceedings of the National Academy of Sciences Colloquium on the roles of homologous recombination in DNA replication are summarized. Current findings in experimental systems ranging from bacteriophages to mammalian cell lines substantiate the idea that homologous recombination is a system supporting DNA replication when either the template DNA is damaged or the replication machinery malfunctions. There are several lines of supporting evidence: (i) DNA replication aggravates preexisting DNA damage, which then blocks subsequent replication; (ii) replication forks abandoned by malfunctioning replisomes become prone to breakage; (iii) mutants with malfunctioning replisomes or with elevated levels of DNA damage depend on homologous recombination; and (iv) homologous recombination primes DNA replication in vivo and can restore replication fork structures in vitro. The mechanisms of recombinational repair in bacteriophage T4, Escherichia coli, and Saccharomyces cerevisiae are compared. In vitro properties of the eukaryotic recombinases suggest a bigger role for single-strand annealing in the eukaryotic recombinational repair.