Intermolecular Renaturation of Hairpin Loops Leads to Stable and Specific Aggregates of 28‐S rRNA and Precursor rRNAs from Xenopus laevis

Abstract

Under certain extraction conditions, 28-S rRNA of Xenopus laevis can form specific dimers by intermolecular renaturation of a particular hairpin loop near the B end of the molecules. This was demonstrated by electron microscopy of isolated aggregates under conditions where rRNA shows a reproducible pattern of secondary structure. In 50% formamide, 15 mM sodium chloride, 1.5 mM trisodium citrate, pH 7.0, the aggregates dissociate into their monomers at a temperature (61 degrees C) at which the most stable hairpin loops of the 28-S rRNA are melting. Precursor rRNAs containing the 28-S rRNA region also form aggregates with 28-S rRNA by the same linkage mechanism. Since such artificial aggregates yield molecular weights higher than expected for true precursors of Xenopus rRNA, they may interfere in precursor rRNA studies. Analysis of whole-cell RNA pulse-labeled with (Me-3H)methionine, or nucleolar RNA pulse-labeled with (3H)uridine in the presence of tubercidin (inhibitor of rRNA processing), as well as electron microscopy of high-molecular-weight RNA fractions confirm that the 40-S precursor rRNA (2.8 X 10(6)) is the primary transcript of the ribosomal genes in Xenopus laevis.