Hemoglobin switching in chickens. Is the switch initiated post-transcriptionally?

We have investigated the transcriptional specificity of chick embryo erythroid nuclei as a function of developmental age during progression of the hemoglobin switch. Nuclei were allowed to transcribe in vitro in the presence of high specific activity [alpha-32P]CTP and the radioactive transcripts were hybridized to a collection of plasmid clones spanning the beta-like globin gene region of the chicken genome. The results reveal locus-specific waves of transcription appearing during the interval between 5 and 12 days of incubation. The last wave, which comes in at 12 days, is highly specialized in transcription of the adult beta-globin gene locus. The most interesting wave of transcription occurs at 6-7 days. The most actively transcribed gene in the early part of this wave is the embryonic rho gene. As the wave progresses, the rho gene activity gradually gives way to beta gene transcription. Definitive red blood cells, which would be synthesizing little rho globin protein at this stage, appear to be responsible for the rho gene transcription. These results and additional data which we present indicate that during the initial stages of the hemoglobin switch the embryonic globin genes are silenced post-transcriptionally.