Topography of the Escherichia coli 30S ribosomal subunit and streptomycin binding

Treatment of Escherichia coli 30S ribosomal subunits with trypsin sequentially removes a number of different ribosomal proteins, as revealed by polyacrylamide gel electrophoresis. Proteins that are removed early by trypsin correlate well with those that are added last during reconstitutive assembly of the 30S subunit from 16S ribosomal RNA and the total protein complement. Proteins that are resistant to removal from the subunit by the highest trypsin concentration used correlate with those that are added early during assembly. Six proteins can be removed from the subunit with trypsin without affecting its ability to bind the antibiotic streptomycin. A decline in the ability of the 30S subunit to bind streptomycin is correlated with the removal of either one, or both, of two proteins, neither one of which is the gene product of the streptomycin locus. The implications of these findings for the topography and assembly of the 30S subunit are considered.