AbstractPrevious studies have indicated that control and hemin‐treated human eryth‐roleukemia K‐562 cells fail to produce adult‐type β‐globin mRNA transcripts and to translate them into nascent β‐globin chains. Expression of the β‐globin DNA sequences in K‐562 cells can occur, however, under certain conditions. To readdress this issue and to examine the possibility of whether these cells produce immature and untranslatable β‐globin RNA transcripts, we prepared total cyto‐plasmic RNA from control and inducer‐treated cells and performed Northern blot hybridization analysis using 5′ end‐labeled fragments of the human β‐globin DNA rather than 3′ end fragments as probes. Although hybridization of both cytoplasmic and nuclear K‐562 RNA with a32P‐labeled 3′ end fragment (1.6kb Bam H1 cut) coding for a large part of the first exon of β‐globin failed to detect β‐globin RNA transcripts, hybridization witha 5′end32P‐labeled 2.0kb Bam H1 fragment (coding for the third exon and part of the second) revealed the presence of relatively small (<7S) RNA molecules both in nuclear and cytoplasmic fraction. S1nuclease mapping of both cytoplasmic and nuclear RNA with the use of 5′ end‐labeled 2.0 kb Bam H1 fragment of human β‐globin DNA indicated protection of a small portion located 64bp 5′ upstream from the Bam H1 site of the second exon. The amount of protected portion was relatively higher in K‐562 cells undergoing erythroid maturation. These findings suggest that control and differentiating K‐562 cells synthesize β‐globin‐like RNA transcripts that are 3′ end short, immature, and unable to give rise to adult β‐globin chains. These results also indicate that K‐562 cells may lack factors that are un
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