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Improvement of Germ Line Transmission by Targeting β‐galactosidase to Nuclei in Transgenic Mice

The Escherichia coli lacZ gene has frequently been used as a reporter in cell lineage analysis, in determining the elements regulating spatial and temporal gene expression, and in enhancer/gene trap detection of developmentally regulated genes. However, it is uncertain whether lacZ expression affects eukaryotic cell growth and development. By using a gene trap, we previously isolated the promoter, Ayu1, which is active in ES cells and in several tissues including the gonads. We used this promoter and the nuclear location signal of the SV40 large T gene to locate β‐galactosidase either in the cytoplasm or the nucleus. Transgenic lines containing β‐galactosidase in the cytoplasm of a wide variety of cell types did not transmit the transgene to their offspring. In contrast, transgenic mice, containing β‐galactosidase in the nucleus, did transmit the transgene successfully. Interestingly, lacZ expression in the brain was more restricted when β‐galactosidase activity was detected in the cytoplasm. These data suggested that cytoplasmic β‐galactosidase affects certain developmental processes or gametogenesis resulting in transmission distortion of the transgene, and that this effect can be reduced by targeting β‐galactosidase to the nucleus. We also found that Ayu1‐driven lacZ expression in the duodenum of adult transgenic mice was sexually dimorphic, being positive in females and negative in males.