A group of eight unique RNA sequences isolated from a single in vitro selec
tion experiment were characterized based on their Diels-Alderase properties
; Five of the eight isolates contain a 10-base conserved region, while the
remaining three do not share any sequence similarities with any of the othe
rs. This series of RNA Diels-Alderases displayed k(cat), and K-m values tha
t spanned 74 and 80-fold ranges, respectively. These values appeared to com
pensate for one another, as the value of k(cat)/K-m varied by only 5-fold a
mong the isolates studied. Product inhibition experiments revealed a genera
l trend between affinity for the cycloaddition product and catalytic abilit
y as measured by k(cat),,t. The activities of each of the RNA Diels-Alderas
es were completely dependent on the presence of cupric ion. In addition, al
l of the RNA catalysts demonstrated a high degree of substrate selectivity,
even for structurally and electronically similar dienophiles. Overall, the
data demonstrate how a variety of RNA sequences can solve a specific chemi
cal problem. The information gathered provides a foundation from which to b
uild our understanding of how sequence, structure, and function are interre
lated. Such knowledge could be applied to create new catalysts, both large
and small.