As conditional genetic strategies advance, the need for multiple site-specific recombinase systems has emerged. To meet this need in part, we have targeted the constitutive ROSA26 locus to create a mouse strain with generalized expression of the enhanced version of the site-specific recombinase FLP (FLPe). This strain is designated FLPeR ("flipper"). Using this strain, extensive target gene recombination can be achieved in most tissue types, including cells of the developing germ line. FLPeR mice therefore serve two important functions: as a source of many different FLPe-expressing primary cell lines and as a deleter strain. Moreover, because the FLPeR mouse is a 129-derived strain, a 129 genetic background can be preserved when crossed to most ES cell-derived mice. This enables conditional genetic alterations to be maintained on a standard background, a feature important for obtaining reproducible results and genetically defined controls.
Sox genes encode proteins related to each other, and to the sex determining gene Sry, by the presence of a DNA binding motif known as the HMG domain. Although HMG domains can bind to related DNA sequences, Sox gene products may achieve target gene specificity by binding to preferred target sequences or by interacting with specific partner proteins. To assess their functional similarities, we replaced the HMG box of Sry with the HMG box of Sox3 or Sox9 and tested whether these constructs caused sex reversal in XX mice. Our results indicate that such chimeric transgenes can functionally replace Sry and elicit development of testis cords, male patterns of gene expression, and elaboration of male secondary sexual characteristics. This implies that chimeric SRY proteins with SOX HMG domains can bind to and regulate SRY target genes and that potential SRY partner factor interactions are not disrupted by HMG domain substitutions. genesis 28:111-124, 2000.
The ultraspiracle (usp) gene encodes a nuclear receptor that forms a heterodimer with the ecdysone receptor (EcR) to mediate transcriptional responses to the insect steroid hormone, 20-hydroxyecdysone (20HE). The responses ultimately elicit changes associated with molting and metamorphosis. Although Ultraspiracle (USP) is required at several developmental times, it is unclear whether USP plays stage-specific roles in Drosophila. A chimeric transgene (d/cusp), produced by replacing the ligand-binding domain (LBD) of Drosophila USP with the equivalent domain from another Diptera, Chironomus tentans, was tested for its ability to rescue Drosophila usp mutants from early larval lethality. A single copy of the d/cusp was sufficient to rescue transformants from several lines through larval development but they died suddenly during the late third instar. Additional doses of d/cusp were required to allow survival through the adult stage, but they did not restore a normal prepupal contraction. Thus, the arrest at the onset of metamorphosis apparently is caused by the impaired ability of the chimeric USP to mediate a stage-specific function associated with the LBD.
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