A previously-isolated ribozyme with capping activity has self-decappin
g activity, here characterized alongside its additional, somewhat para
llel, pyrophosphatase reaction. Decapping is 10-50 times slower than t
he pyrophosphatase activity, depending on pH. The RNA accelerates pyro
phosphate release 170 000 times over a control composed of randomized
pppRNA, and 5' capped RNA accelerates decapping 1000-fold over random
capped RNA. Triphosphate-linked G(S')pppRNA also supports an unusual c
ap-exchange reaction, exchanging its cap with guanosine 5'-tetraphosph
ate to form pentaphosphate-linked G(S')pppppRNA. GDP, a capping reacta
nt for the RNA, appears to suppress both decapping and pyrophosphatase
activities. Autodecapping and pyrophosphatase activities have in comm
on an unusual divalent metal ion requirement for Ca2+ or less effectiv
ely Mn2+, and both are active over a broad pH range of 4.5-9.0. These
characteristics resemble the capping activity of the same RNA. Kinetic
analysis reveals a well-defined Ca2+-RNA complex, and Mg2+ and Sr2+ a
ct as competitive inhibitors of Ca2+. A strong Ca2+-binding site is su
ggested by a low K-M Of 40-60 mu M at pH greater than or equal to 7.0.
The role of Ca2+ in these reactions can be surmized from literature d
ata on reactivity of nucleotide phosphates. Pyrophosphatase, capping,
and decapping activities of isolate 6 RNA are apparently carried out b
y a single reaction center, whose rate of reaction with all nucleophil
es sums to a constant total rate. This suggests a universal rate-limit
ing step. Versatile activation of alpha-phosphate by this reaction cen
ter raises the possibility of combinatorial ribozymes.