15
Eukaryotic RNA Polymerases
Background
There are several recent review articles on eukaryotic RNA polymerases
(47-52). These may be consulted for discussions that are omitted in this
review. The DNA dependent RNA polymerase activity was first demonstrated
in rat liver nuclei by Weiss in 1955 (103) who partially purified the en
zyme which was firmly attached to DNA. Mans and Novelli (104) were the
first to study a solubilized eukaryotic RNA polymerase in 1964. However
the problem of freeing the RNA polymerase from DNA was more severe for the
other eukaryotic RNA polymerases studied and consequently most studies on
nuclear RNA polymerase activity were carried out on isolated nuclei or
unpurified chromatin until a eukaryotic RNA polymerase was purified in
1965 (105).
Isolated nuclei demonstrated transcription activity which synthesized
mostly GC-rich ribosomal RNA at low ionic strength. This activity was
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localized in the nuceolus and stimulated by Mg However, at high ionic
strength mostly DNA-like RNA was synthesized by an activity which was
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stimulated by Mn and localized in the nucleoplasm (106-111). The nucleo-
plasmic activity was specifically inhibited by a-amanitin (3).
Multiple forms of nuclear RNA polymerases were demonstrated by Roeder
and Rutter (5) in rat liver and Xenopus laevis. They demonstrated three
types of eukaryotic RNA polymerases which could be separated based on
chromatographic properties, sensitivity to specific inhibitors and sensi
tivity to ions. The first RNA polymerase eluted from a DEAE Sephadex
column (nuclear RNA polymerase I) was found to be of nucleolar orgin,
insensitive to a-amanitin and synthesized a product which hybridized to