Unexpected nuclear localization of a chimeric beta-galactosidase lacZ reporter gene product in mammalian cells.

A lacZ cassette was designed to include a synthetic amino terminus optimized for translation in eukaryotic cells, as well as multiple restriction sites for the insertion of heterologous regulatory elements both 5' and 3' of the reporter. The cassette was placed under the control of the metallothionein promoter in combination with the SV40 enhancer and this plasmid was introduced into mammalian cells. High levels of beta-galactosidase were observed in several cell types, demonstrating efficient expression of the reporter. Unexpectedly, most of the chromogenic reaction product appeared to be intra- or peri-nuclear, indicating that the enzyme is similarly localized. The synthetic amino terminus does not resemble known nuclear localization signals and thus may constitute a novel signal.