Specific in situ Cleavage of 16S Ribosomal RNA of Escherichia coli Interferes with the Function of Initiation Factor IF-1

Specific in situ cleavage of 16S rRNA of E. coli has been accomplished by in vitro treatment of 70S ribosomes (``tight couples'') with the bacteriocin cloacin DF13. The defective ribosomes, which have fully lost their ability to sustain polypeptide synthesis, are still able to form initiation complexes with MS2 RNA, but the kinetics are altered. This is apparently due to an improper functioning of initiation factor IF-1, for the defective ribosomal couples respond normally to dissociation by IF-3 but the dissociation is not stimulated by IF-1. The initiation complexes formed with defective ribosomes are fully reactive with puromycin. Their ability to bind alanyl-tRNA is reduced by about 50% at all concentrations of elongation factor Tu studied. Cleavage of the 16S rRNA, not the release of the terminal fragment from the ribosome, causes the block of protein synthesis and the aberrations observed during initiation and elongation.