Recombinant <i>ubbR</i> promoter enables highly efficient tamoxifen-inducible Cre recombination in embryonic and adult zebrafish

The ability to inactivate gene function in an adult organism is essential for studies of biological processes such as regeneration and behavior. This is best achieved by engineering an allele that could be conditionally inactivated using drug-inducible Cre recombinase. Several recent studies clearly demonstrate the feasibility of engineering such conditional alleles in zebrafish. Meanwhile, achieving a sufficient degree of recombination to induce complete loss of function has remained a major limitation. Herein, we address this limitation by engineering a recombinant ubiquitin promoter ubbR consisting of the zebrafish ubiquitin promoter supplemented with an intronic enhancer from the carp beta-actin2 gene. Using phiC31-mediated targeted integration, we demonstrate that ubbR outperforms both parental promoters at all embryonic stages tested. Furthermore, the ubbR:CreERT2 driver line we generated ensures a high-level recombination of floxed alleles in adult zebrafish tissues. Finally, we demonstrate that our ubbR promoter-driven construct retains high activity when integrated at other genomic loci, making this promoter a promising new tool for robust expression of transgenes at all stages of zebrafish ontogenesis.