Expression of adult globin mRNAs in hamster yolk-sac erythroid cells during ontogeny

The embryonic-to-adult globin switch begins in intact yolk-sac erythroid cells (YSEC) of hamster embryos 2 days after initiation of erythropoiesis in the yolk-sac blood islands ("early" ontogeny), and is nearly completed 4 days later ("late" ontogeny). Rates of adult globin-chain synthesis, alpha and beta major, in intact YSEC are low in early ontogeny and peak in late ontogeny. The opposite phenomenon is observed when RNA procured from YSEC during the same ontogenic time interval is translated in cell-free wheat-germ extract. That is, peak rates of adult globin chain synthesis are observed in early ontogeny and decline thereafter. This information led us to search for, and find, a translational regulator, apparently protein, in YSEC cytoplasm that suppresses translation of adult globin mRNA in early ontogeny, thus explaining the discordant in vivo (intact cells) and in vitro (cell free) rates of synthesis of adult globins. To further characterize the globin switch at the level of RNA transcription, we quantified adult globin mRNAs (alpha and beta major) from YSEC on consecutive days of gestation by nuclear runoff, slot blot and Northern blot assays. Results of nuclear runoff assays showed maximal rates of transcription of alpha and beta major globin mRNAs in early ontogeny which gradually declined to near zero by late ontogeny. Results of slot blots and Northern blots showed that amounts of alpha and beta major globin mRNAs per YSEC were high in early ontogeny and gradually declined to near zero by late ontogeny. It is apparent that the embryonic-to-adult globin switch in intact YSEC correlates neither with the transcriptional activity of the adult globin genes nor with rates of adult globin-chain synthesis as assayed by cell-free translation of globin mRNA. These studies now provide information at the transcriptional level (1) complements the cited translational repressor work and (2) demonstrates that adult globin transcription and translation are pronouncedly discordant in hamster YSEC undergoing globin-chain ontogeny in the embryonic circulation.