CfMV frameshift signals generated significant À1 PRF in yeast. À1 PRF level measured from pAC-A was 3-fold higher than from HIV RNA (Figure 2A and B) . The extent of À1 PRF directed by the minimal region A in yeast, 14.4 -1.9%, was at the same level as that reported for the CfMV minimal frameshift signal in vitro (12.7%) (18) . In contrast to our earlier in vitro observations (18) , the longer CfMV sequences upregulated À1 PRF in vivo. In yeast, the level of upregulation was 2-fold for pAC-B, the À1 PRF frequency being 26.3%, and almost 5-fold for pAC-C resulting in efficiency close to 70% (Figure 2A) , which is an extremely high value, if compared to the other values published earlier (3). CfMV frameshift signals directed À1 PRF at a lower level in bacteria than in yeast ( Figure 2B ). The extent of À1 PRF directed by region A in bacteria was 2.4 -0.7%. As in yeast, the longest B region stimulated À1 PRF 2-fold in bacteria when compared with pAC-A. However, region C did not further improve À1 PRF, but programmed À1 PRF to similar levels as pAC-B, the percentages being 4.7 -1.6% for pAC-C and 5.5 -1.5% for pAC-B.