Evidence for translation of HPRT enzyme on maternal mRNA in early mouse embryos.

This paper presents evidence that maternal mRNA is responsible for the early increase in HPRT activity in preimplantation mouse embryos. Increase of HPRT activity is demonstrable from as early as 6 h postfertilization when there is barely detectable synthesis of embryonic RNA. The increase is sensitive to cycloheximide and thus requires protein synthesis, whereas it is insensitive to alpha-amanitin and therefore independent of mRNA synthesis. These results suggest that translation of HPRT occurs on pre-existing maternal mRNA. Embryo-coded HPRT activity is detectable by the 4- to 8-cell stage when the increase in HPRT activity becomes sensitive to alpha-amanitin. The transition from maternal- to embryo-coded enzyme activity is completed by the time of compaction. At this stage there is an unexplained yet reproducible loss of HPRT activity. Other maternally-inherited enzymes show a marked degradation occurring at a similar time. It is possible that the enzyme degradation observed reflects some common mechanism directing the changeover from maternally-derived to embryonically-derived enzymes.