Substructure of Nuclear Ribonucleoprotein Complexes

Nascent heterogeneous nuclear RNA (hnRNA) synthesized in regions of active chromatin rapidly becomes associated with protein; the presence of protein on the growing RNA chains has been visualized in spread chromatin by electron microscopy (EM) (see, e.g., Miller and Bakken 1972; Malcolm and Sommerville 1974; Laird et al. 1976; McKnight et al., this volume). It is reasonable to presume that this protein folds the lengthening RNA chains, facilitates their removal from the template, and later participates in the processing and maturation events which lead to the turnover of nucleus-restricted sequences while mature mRNA molecules are transported to the cytoplasm. There have been very extensive studies on the pathway of nuclear RNA molecules from synthesis to the appearance of mRNA on cytoplasmic polyribosomes. It is fair to say that, at present, the wealth of information produced promises rather than achieves a satisfying description of this complex process. These studies have centered...