Two Forms of the 30 S Ribosomal Subunit of Escherichia coli

Two forms of the 30 S ribosome subunit were separated by agarose acrylamide gel electrophoresis. The slower migrating form was associated predominantly with native 30 S and run-off 70 S ribosomes, whereas the faster migrating form was associated with polyribosomes. The two forms were separated by sucrose density gradient centrifugation and characterized. The primary difference between the two forms appears to reside in the presence of ribosomal protein S1 in the slower migrating form. The forms are interconvertible by the addition or removal of S1. Addition of messenger RNA influences this conversion. It is proposed that mRNA binds to S1-containing ribosomes, forming a complex at 0° in the absence of initiation factors. The specificity of this complex is suggested by the fact that tRNA and rRNA cannot substitute for mRNA. Furthermore, the partial disruption of the secondary structure of Qβ RNA by mild formaldehyde treatment increases the ability of this RNA to bind the S1-containing ribosomes. The affinity of S1 for the ribosome is weak and can be altered by interaction with specific drugs as well as by variations in salt concentrations. S1 is removed from the ribosome by colicin E3 treatment indicating that S1 binds near the 3′ end of 16 S rRNA within the region of the 50 nucleotides cleaved by colicin E3. The data suggest that within the cell there is probably one S1 protein associated with each 30 S subunit.