THE EXPRESSION OF INTRACISTERNAL-A PARTICLE GENES IN THE PREIMPLANTATION MOUSE EMBRYO

Intracisternal A particles (IAP), murine endogenous retrovirus, make up 0.3% of the mouse genome. They are expressed in some normal tissues, certain transformed cell lines, and show stage-specific patterns of expression in early embryos. We have used peptide-specific antisera and the polymerase chain reaction to explore type-specific expression of these IAP during preimplantation development. In this paper we show that the IAP core protein, p73, characteristic of type I IAP, is present throughout preimplantation development while the gag-pol fusion protein p120, characteristic of the variant type IΔ1, is synthesized and expressed only from the 8-cell stage onward. Type I IAP RNA is present at all stages and appearance of p120 at the 8-cell stage could represent new transcription or translation from a preexisting IΔ1 message. The presence of type II IAP RNA varies according to stage, with two sizes of type II transcripts present at all stages except the 2-cell stage at which time only the smaller of the two transcripts can be detected. The reappearance of the larger type II transcript subsequent to the 2-cell stage implies new transcription of this type II subspecies. The presence of type I, II, and p73 in the unfertilized egg strongly suggests maternal inheritance from the oocyte. © 1991.