Analysis of the Adult Chicken Beta-globin Gene. Nucleotide Sequence of the Locus, Microheterogeneity at the 5'-end of Beta-globin MRNA, and Aberrant Nuclear RNA Species

We have determined the nucleotide sequence of the adult chicken beta-globin gene locus. The structure of the adult chicken beta-globin gene is, for the most part, analogous to that of mammalian beta-globin genes thus far examined. The gene occupies 1525 nucleotides (nt) from the major cap site to the poly(A) addition site. Introns of 92 and 810 nt interrupt the coding sequence at positions corresponding to amino acid codons 30/31 and 104/105. The regions of most significant homology outside of the transcribed sequence occur within the 5'-untranslated region where the CCAAT and ATA boxes are found -74 and -30 nucleotides from the major chicken beta-globin mRNA CAP site. Using both primer-extension and mung bean nuclease mapping to localize the 5' terminus of the adult beta-globin mRNA within the transcription unit of this gene it was found that 5'-untranslated region is heterogeneous in length with termini 80 and 83 nucleotides from the translational initiation codon. In examining the transcription unit of this gene and the processing of its RNA species during maturation to mRNA, three distinct classes of beta-globin complementary nuclear RNAs are observed. One class of RNA consists of species whose size ranges from 1750 to 900 nt. These RNAs contain all or portions of the introns and are polyadenylated. Although the introns appear to be removed in a multistep fashion, a single kinetic processing pathway is not obvious. An additional processing pathway which leads to the accumulation of stable aberrant RNAs is also observed. This second class of RNA species contains first intron sequences, but lacks portions of the beta-globin coding sequence and thus is too small to become processed to functional mRNA. The third class of beta-globin complementary RNAs appear to migrate during gel electrophoresis at positions equivalent in size to RNAs of greater than gene length (2400-4500 nt), but are probably also never processed into mature mRNA.