A well transformable E. coli tdk-strain--suitable for direct rescue of tk gene plasmids from mammalian cells.

An E. coli C600 tdk strain which is suitable for the direct rescue of the eukaryotic HSV-1 tk gene by functional selection in E. coli is described. This E. coli tdk mutant failed to grow on fluorouracil selection plates and showed values for thymidine kinase activity smaller than in the tdk negative KY 895 strain. It is genetically well transformable, i.e. the transformation efficiency with plasmid DNA from pSK1 amounts to 5 X 10(6) or 3 X 10(8) depending on the transformation procedure applied.