The <I>in situ</I> Occurrence in Nuclei of 5'-and 3'-Terminal Hairpin Structures of Chicken Reticulocyte Nuclear RNA

Abstract

In situ secondary structures of chicken reticulocyte heterogene-ous nuclear RNA (HnRNA) were investigated using a nucleic acid intercalating agent.Double-stranded regions, if present in situ, of chicken reticulocyte HnRNA were cross-linked by treating isolated reticulocyte nuclei with the psolaren derivative, 4'-hydroxymethyl-4, 5', 8-trimethyl psoralen and ultraviolet irradi-ation. HnRNA was then isolated and examined by electron microscopy under stringent denaturation conditions. At least four different shapes of molecules were seen: a) filamentous forms without any loops or fold back structures at the ends, or b) with a small loop (or fold-back structure) at one end only and c) large or medium sized loop-bearing molecules without or d) with a small loop (or fold-back structure) at one end.It is suggested that small and large loops found in the above shapes of molecules represent the 5'-and 3'-terminal double-stranded hairpin structures, respectively. This result implies an in situ occurrence in nuclei of the intra-molecular hairpin structure in chicken reticulocyte HnRNA molecules.It was estimated that intervening sequences 1 and 2 of chicken β-globin 'pre-mRNA' would be 113-120 and 671-690 nucleotides long, respectively.