Analysis of thymidine kinase gene expression in preimplantation mouse embryos

Thymidine kinase (TK) activity was examined during the development of preimplantation mouse embryos. TK activity was increased approximately 20‐fold from day 2 embryos (2‐cell) to day 5 embryos (late blastocyst). TK activity did not change along with the progression into S‐phase of the first and the second cell cycles but increased sharply at S‐phase of the third cell cycle. Analysis of TK mRNA with a reverse transcription‐polymerase chain reaction (RT‐PCR) method showed that the level of TK mRNA was low in ovulated eggs and 1‐cell embryos and was hardly detectable in day 2 embryo (2‐cell), but sharply increased in day 3 embryos (mixture of 5‐ to 8‐cell and morula). The functional role of 5′‐flanking sequence of TK gene was also investigated in preimplantation embryos after microinjection with the DNA construct of 5′‐flanking sequence of TK (2.4 kb) linked to bacterial lacZ gene (TK2.5lacZ) into the pronucleus of 1‐cell and subsequently by histochemical staining with X‐gal. β‐Galactosidase activity was first detected in day 3 embryos (8‐cell), and 30% of embryos were stained with X‐gal in day 4 and day 5 embryos, respectively. These results show that an increase in TK activity occurred after 2‐cell stage, and this increase was primarily due to the embryonic activation of TK gene expression. Also, it appears that the 5′‐flanking sequence of TK may directly regulate the TK gene expression at the transcriptional level during preimplantation murine development. © 1994 Wiley‐Liss, Inc.