Translating ribosomes can shift reading frame at specific sites with high efficiency for gene expression purposes. The most common type of shift to the −1 frame involves a tandem realignment of two anticodons from pairing with mRNA sequence of the form X XXY YYZ to XXX YYY Z where the spaces indicate the reading frame. The predominant −1 shift site of this type in eubacteria is A AAA AAG. The present work shows that in Escherichia coli the identity of the 6 nt 3′ of this sequence can be responsible for a 14-fold variation in frameshift frequency. The first 3′ nucleotide has the primary effect, with, in order of decreasing efficiency, U > C > A > G. This effect is independent of other stimulators of frameshifting. It is detected with other X XXA AAG sequences, but not with several other heptameric −1 shift sites. Pairing of E. coli tRNALys with AAG is especially weak at the third codon position. We propose that strong stacking of purines 3′ of AAG stabilizes pairing of tRNALys, diminishing the chance of codon:anticodon dissociation that is a prerequisite for the realignment involved in frameshifting.