. Chem. 258:3991-3995, 1983). We found that I-globin RNA is assembled into heterogenous nuclear RNA-protein complexes in a specific manner. There are several regions of nuclease resistance in the first and third exons interrupted at regular intervals by sensitive regions. The second exon has only one nuclease-resistant region. The resistant regions range in size from 20 to 50 nucleotides. This organization may reflect a specific mode of assembly for heterogenous nuclear RNA-protein complexes.Concomitant with the transcription of heterogenous nuclear RNA (hnRNA), the transcript is bound to proteins (14,20) to form heterogenous nuclear RNA-protein complexes (hnRNP). When precautions are taken to limit the action of endogenous nucleases, the isolated hnRNP can be quite large (200S) and have a beaded appearance in electron micrographs (12,21,31). When either endogenous or exogenous nucleases are allowed to act on the hnRNP, particles between 30 and 50S are released. These core particles are considered to be the basic structural units of hnRNP (13,16), and although there may be as many as 40 protein components of these particles, there are usually only six major proteins, with molecular weights between 30,000 and 45,000 (3,10,16,26,33). These major proteins can be cross-linked by UV light to RNA in situ (5); therefore, the interaction between hnRNA and protein is apparently close.Since the assembly of hnRNA into hnRNP takes place while transcription is occurring, the subsequent steps in the maturation of the RNA must take place with the proteins attached to the transcript. These proteins might serve to protect the RNA from digestion by nucleases. Alternatively, these proteins could stabilize (1) or actually induce (35) an RNA structure which is favorable for processing or transport. A central question regarding the structure of hnRNP is whether the proteins are bound to the RNA in a specific manner. Beyer et al. (4) provided visual evidence, in the form of electron micrographs, that hnRNP are assembled nonrandomly. There is biochemical evidence supporting the specific assembly of hnRNP (23,28,32,34). In all of the related biochemical studies, nuclease digestion combined with analysis of nuclease-resistant RNA has been the basic approach, with the assumption that interaction with proteins affords resistance to nuclease digestion. Solution hybridization (28) and Southern blot hybridization (22,23,32) ever, sensitivity of these methods is low and at best can only define a general area that is resistant to nuclease.We found that 3-globin RNA is assembled into hnRNP in a specific manner in chicken reticulocytes. Nuclease-resistant regions have been mapped, at the resolution of nucleotides, by the new RNA mapping method we have recently developed (25). There are extensive regions of nuclease resistance in the first and third exons, interrupted at regular intervals by sensitive regions. The second exon has only one nuclease-resistant region. We discuss how this organization could reflect a specific mode of assembly for hnR...
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