Efficient expression of Escherichia coli galactokinase gene in mammalian cells.

The Escherichia coli galactokinase gene (galK) was inserted into a modified early region transcription unit of simian virus 40 (SV40) contained on a bacterial plasmid. Introduction of this pSVK vector into monkey, mouse, and hamster cell lines by transfection resulted in efficient expression of the bacterial galK gene. This expression was shown to be dependent upon fusion of the galK gene to the early promoter of SV40 and did not appear to require SV40 splice signals. Moreover, expression in these cells could be obtained either transiently, 24--72 hr after transfection, or continuously, after stable transformation. In particular, pSVK-dependent galK expression was obtained in a hamster cell line genetically deficient in galactokinase activity. Expression of the bacterial enzyme was shown to complement the galactosemic defect of these cells, thereby allowing their selective survival and growth on galactose as the only carbon source. The ability to readily assay, select for, and potentially select against galK expression from pSVK and its derivatives should prove extremely useful in studying eukaryotic gene regulatory signals.