ENDOGENOUS TRANSCRIPTION OCCURS AT THE 1-CELL STAGE IN THE MOUSE EMBRYO

In most animal species, the earliest stages of embryogenesis are regul ated by maternally inherited components, at least until the activation of the zygotic genome, Although the first proteins derived from the a ctivation of the embryonic genome have been detected long ago at the 2 -cell stage in the mouse, the exact timing of transcriptional activity resumption after fertilization is still a matter of debate. Any new i nformation about this critical event is relevant to the practice of tr ansgenesis and cloning of embryos as well as to the general understand ing of the regulation of nuclear processes following fertilization. Us ing a new fluorescent method allowing the detection of in vivo RNA syn thesis, we show that endogenous transcription by RNA polymerase II tak es place unambiguously as early as at the late 1-cell stage, Furthermo re, we demonstrate that transcription is first initiated in the patern al pronucleus. (C) 1995 Academic Press, Inc.