Further evidence that the ribosomal 30S proteins S3, S5, S9, S11, S12, and S18 possess specific 16S RNA binding sites.

E. Coli ribosomal 16S RNA prepared by an acetic acid-urea extraction technique individually binds, in addition to the seven established proteins, 6 new 30S ribosomal proteins (S3, S5, S9, S12, S18 and S11) (Hochkeppel et al., 1976). In this communication we demonstrate the site specificity of these proteins. Binding curves of the individual proteins with acetic acid-urea 16S RNA show that the binding of all six proteins to the RNA reaches a plateau at 0.3-0.97 copies per 16S RNA molecule. No significant binding of these proteins to classicial phenol extracted 16S RNA is observed, with the exception of S13 which binds 0.2 copies of protein per molecule of 16S RNA. Specificity of binding of these proteins is also demonstrated in "chase" experiments. The site specificity of individual [3H]-labeled 30S proteins bounds to 16S RNA is tested by the addition of non-radioactive 30S total protein to the reaction mixture.