Globin mRNA in Friend cells: its structure, function and synthesis.

Accumulation of haemoglobin in induced MEL cells begins with the activation of transcription of the globin genes. Though much has been learned of cellular events affecting expression of the globin genes, e.g., from noninducible variants of MEL cells and cell fusion between MEL cells and other cell types, there is at present no in vitro system available that would permit more detailed study of the molecular events leading to transcription of the globin genes. Presumably, with the availability of cloned chromosomal genes such systems will soon be found. The products of transcription detected in induced MEL cells are 15 S and 11 S species which are precursor forms of beta- and alpha-globin mRNA, respectively. An unmodified primary transcript has not been detected. The 15 S species possesses a fully methylated 'cap' 1 structure the 5' end and poly(A) at the 3' end. Conceivably, there could be cleaving or splicing events preceding the 'capping' and polyadenylation, but all these reactions must occur extremely rapidly, since with a t 1/2 approx. 2 min a large proportion of the 15 S beta-globin RNA must be newly synthesised. It also contains the two intervening sequences found in the chromosomal genes. Selective processing occurs since from pulse and pulse-chase experiments most if not all of the 15 S beta-globin RNA is processed to mature 10 S beta-globin RNA very rapidly, whereas less than 10% of newly synthesised nuclear RNA (HnRNA) leaves the nucleus, the remainder being hydrolysed in the nucleus with a t 1/2 approx. 20 min. Perhaps rapid processing permits efficient transport to the cytoplasm. Further processing occurs in steps; apparently the large intervening sequence is removed first followed by the small intervening sequence. These steps do not appear to be rate limiting events and these sequences have not been detected separately from the 15 S beta-globin RNA. Such results and the wide divergency of intervening sequences, suggest that the intervening sequences per se play no essential function in the cell, though their presence in the nuclear transcript appears to necessary for processing to the mRNA. The selective processing accounts for more increase of the globin RNA in MEL cells, and further accumulation occurs by virtue of the stability of globin mRNA (t 1/2 approx. 17 h) compared with the bulk of poly(A)-RNA (t 1/2 approx. 3 h). It would appear, however, that specific destabilization of a class of stable mRNA (t 1/2 approx. 35 h) is necessary to allow globin mRNA to asccount for 90% of the mRNA population in reticulocytes.