The following experiments were done in a strain carrying deletion of SPA (high polyamines) because it gives higher frameshifting and higher b-galactosidase activity in general; however, we obtained similar ratios for mutant to wild-type frameshifting ef®ciency in a strain with the intact SPA gene. Deleting 5¢ sequences up to the third to last sense codon of ORF1 has little or no effect on frameshifting ef®ciency. Deleting all but the last sense codon (UCC) of ORF1 leads to a 4-to 5-fold reduction in frameshifting ef®ciency ( Figure 5A ). This implies that the conservation of the six nucleotides 5¢ of the UCC-UGA frameshift site is due to their importance for stimulating +1 frameshifting. It also suggests that no additional ORF1 sequences of SPA stimulate the +1 recoding event. The 180 nucleotide 3¢ region was searched for possible structure by computer RNA folding algorithms plus visual inspection. The algorithms predicted several minimal structures in that region. 3¢ deletion constructs (constructs del.3,3¢±81,3¢) tested the importance of any putative structure on the frameshifting ef®ciency. The results ( Figure 5B and C) show that all of these deletions lead to a signi®cant (~10-fold) reduction in +1 frameshifting, indicating the presence of a major 3¢ stimulatory element in the 180 nucleotide region immediately following the frameshift site of SPA. However, the results indicate that none of the putative RNA structures in this region are suf®cient for the activity of this element. Several additional 3¢ deletions delineated the boundaries of this stimulatory element from the frameshift site to 150 and 180 nucleotides downstream (since construct del.150,3¢ stimulates 5.5-fold more +1 frameshifting than del.129,3¢, 150 nucleotides downstream probably contain most of the 3¢ stimulator).