[The 5S rRNA-protein complex of Escherichia coli studied by carbodiimide modification]

5S rRNA-protein complex has been reconstituted from 5S rRNA and total protein of large (L) ribosomal subunit of Escherichia coli. The complex consists of 5S rRNA and 3 proteins only: L5, L18, L25. A water-soluble carbodiimide [N-cyclohexyl-N'-(2-morpholinoethyl)-carbodiimide-methyl-p-toluolsulp honate] cross-links L18 to 5S rRNA at pH 7.2 and L25 to 5S rRNA at pH 7.7. This pH-dependence of cross-linked proteins is a consequence of the difference in stability of the initial complex: the complex has all three proteins at pH 7.7 but L18 mainly at pH 7.2. It has been shown that L18 stimulates the chemical modification of U87 and U89 residues of 5S rRNA by carbodiimide. A model of L18-5S rRNA complex has been proposed.