A Possible Role for 5 S rRNA as a Bridge Between Ribosomal Subunits

18 S rRNA is a high molecular weight polyribonucleotide found in the small subunit, and 26 S rRNA is a high molecular weight polyribonucleotide found in the large subunit, whereas 5 S rRNA and 5.8 S rRNA are low molecular weight ("satellite") polyribonucleotides confined to the large subunit of wheat-embryo ribosomes. Under the same conditions in which 5.8 S rRNA is known to complex efficiently and preferentially with 26 S rRNA, it has been observed that 5 S rRNA complexes efficiently and preferentially with 18 S rRNA. Since 5 S rRNA is a component of the large ribosomal subunit, but it complexes preferentially with 18 S rRNA, which is a component of the small ribosomal subunit, it has been proposed that 5 S rRNA may serve as a "bridge" to mediate reversible association between the small and large ribosomal subunits. The possible role that a polycistronic precursor of rRNA might be visualized to play in the biogenesis and assembly of reversibly associating ribosomal subunits is alluded to in the present context.