Identification of the collar-like structure of the 30S ribosomal subunit from E. coli by dark field electron microscopy.

We have used dark field electron microscopy to study a fragment of the small (30S) subunit of the E. coli ribosome. This fragment is almost the same size as the parent particle but RNA sequencing studies have shown it to lack, as a major constituent, a 150-nucleotide stretch at the 3' end of the rRNA, and two minor sections constituting 20 nucleotides from the 5' end and the 15 nucleotides of the sequence 687-701. The protein composition of the fragment was essentially unchanged. Samples of this material, and controls, were examined in the electron microscope after treatment with a buffered uranyl acetate solution for positive staining. Careful comparison revealed the following differences. The structural feature that we call the "collar" was missing in the fragment. Of the three parallel uranyl-staining bands that we have observed in micrographs of whole 30S subunits, the fragment consistently lacked the uppermost band. These observations identify the top uranyl-adsorbing band as being the 3' end of the ribosomal RNA and show that it can be equated with the collar-like structure.