“…The "Chimeric SELEX" method described here simulates random recombination among functional RNAs derived from RNA populations with 70 or 80 positions of random sequence+ Specifically, we make use of previously described RNA populations that bind chloramphenicol [Cam ], adenosine , or coenzyme A [CoA (Burke & Hoffman, 1998)]+ These aptamers were selected from random initial pools that contained either 70 ("70N") or 80 ("80N") positions of random sequence+ Among the diverse sequences from the Cam-binding populations ("70Cm" and "80Cm"), there were some that contained the motif in Figure 1A, which has a weak but suggestive similarity to the portions of 23S rRNA involved in Cam binding and peptide bond formation )+ All of the isolates from the adenosine-binding population (80S), which was selected to recognize S-adenosyl methionine (SAM), conformed to the sequence motif shown in Figure 1B+ This element has also been isolated in selections for RNAs that bind ATP (Sassanfar & Szostak, 1993) and NAD ϩ (Burgstaller & Famulok, 1994), and it recognizes many other adenosine derivatives+ Of the two CoA-binding populations, the "80CoA" sequences were highly diverse, whereas the "70CoA" population was dominated by a motif that recruited secondary structural elements and 12 specifically required unpaired nucleotides from the primer binding sites (Fig+ 1C)+ This RNA element also binds many adenosine derivatives, but its specificity differs significantly from that of apta-FIGURE 1. Schematic representation of some of the aptamer modules used in this study+ A: One of the Cam-binding aptamer motifs present in both population 70Cm and 80Cm + B: Conserved adenosine-binding aptamer motif present in population 80S (Sassanfar & Szostak, 1993;Burgstaller & Famulok, 1994;+ C: Conserved CoA-binding aptamer motif found in population 70CoA (Burke & Hoffman, 1998)+ N, any nucleotide; n, its base pairing complement; x, variable number of nucleotides of any sequence+ Nucleotides that were intolerant of mutations or were highly conserved are in bold print+ mers from the 80S population+ The 70CoA and 80CoA populations also contained low frequencies of RNAs that required intact CoA for elution from the CoA resin and were not eluted by AMP alone (Burke & Hoffman, 1998)…”