Recognition and alignment of homologous DNA sequences between minichromosomes and single-stranded DNA promoted by RecA protein

Rad54 protein is targeted to pairing loci by the Rad51 nucleoprotein filament

Rad51 and Rad54 proteins are important for the repair of double-stranded DNA (dsDNA) breaks by homologous recombination in eukaryotes. Rad51 assembles on single-stranded DNA (ssDNA) to form a helical nucleoprotein filament that performs homologous pairing with dsDNA; Rad54 stimulates this pairing substantially. Here, we demonstrate that Rad54 acts in concert with the mature Rad51-ssDNA filament. Enhancement of DNA pairing by Rad54 is greatest at an equimolar ratio relative to Rad51 within the filament. Reciprocally, the Rad51-ssDNA filament enhances both the dsDNA-dependent ATPase and the dsDNA unwinding activities of Rad54. We conclude that Rad54 participates in the DNA homology search as a component of the Rad51-nucleoprotein filament and that the filament delivers Rad54 to the dsDNA pairing locus, thereby linking the unwinding of potential target DNA with the homology search process.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Targeted gene disruption: applications in neurobiology

In classical gene inactivation approaches by homologous recombination in embryonic stem cells, the resulting knockout mice are genotypically homogeneous. The inactivation of a gene in the complete organism may sometimes lead to early embryonic lethality. The observation that bacterial recombinases can drive site-specific recombination in mammalian cells has allowed for spatiotemporally controlled genetic modifications. Thus, conditional gene inactivation can be achieved in a specific subset of cells, leaving the rest of the organism genotypically unchanged. Another application of bacterial recombinases is the generation of exon-specific knockout mice, allowing for the analysis of the role of tissue-specific splice variants. A combination of the above-mentioned bacterial recombinase technique with inducible promoter systems permits the investigator to choose precisely the onset of recombination. An extension of the above-mentioned techniques is the combination of the bacterial recombinase technique with adenovirus-based technology, which would open vast possibilities of tissue-specific genetic modifications in a controlled time frame.