The cleavage of 16S ribosomal RNA (rRNA) from E. coli with ''hammerhea
d'' ribozymes and with RNase H in the presence of ''chimeric'' (2'-deo
xy-F-thymidine containing) oligonucleotides has been studied. The cond
itions for the cleavage of a desired single internucleotide bond in th
ese large molecules with very complicated secondary and tertiary struc
tures have been found.