The abundance and stability of erythropoietin (EPO) receptor mRNA was measured in Friend virus-infected mouse splenic erythroblast (FVA) cells. FVA cells correspond to the colony-forming unit-erythroid (CFU- E) and cluster-forming unit-erythroid stages of differentiation and they mature in vitro into reticulocytes during 45 to 60 hours of culture. After 20 hours, there was a rapid decline in the EPO receptor mRNA level through the 45-hour culture period. Levels of actin and beta- globin mRNAs were monitored as two representative proteins that are actively synthesized in early versus late stages of terminal erythroid differentiation. A general decrease of actin mRNA level was apparent, whereas globin mRNA levels increased throughout the culture period. The greatest decrease in both EPO receptor and actin mRNA was observed when the cell population was at the stage of basophilic and polychromatophilic erythroblasts. The half-life for EPO receptor mRNA was approximately 75 minutes, as determined using the transcriptional inhibitor actinomycin D. The half-life measured at several times during the 48-hour culture period remained constant. These results indicate that EPO receptor mRNA must be transcribed continuously until late in the maturation process of FVA cells in order to maintain the levels seen by Northern analysis. The number of copies of EPO receptor mRNA at 0 hours was determined to be 25 copies per cell. This low number and the decline of EPO receptor mRNA correlate with the low receptor numbers on FVA cells, as well as the decline of binding sites with cell maturation.

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