Questions, however, remain about the structure of the HIV-1 frameshift signal when ribosomes are present and bound at or downstream of the slippery sequence. Here, using Escherichia coli ribosomes, we address this question and probe possible interactions of the HIV-1 frameshift signal with the ribosome by using toeprinting and chemical probing assays. The ability of this eukaryotic mRNA frameshifting signals to promote -1 PRF in the prokaryotic translational machinery has been previously demonstrated (33, 34) .