20
The molecular weight of the enzyme, consisting of a single polypep
tide chain, seems to be about 60,000 (11-15). The purified mitochondrial
RNA polymerases from Neurospora crassa (11) and rat liver (12,13,147) are
inhibited by rifampicin in contrast to the mitochondrial enzymes from
Xenopus laevis (14), wheat leaf (150) and Ehrlic Ascites (39) which are
resistant to rifampicin. Some investigators have found the yeast mito
chondrial RNA polymerase to be rifampicin resistant (15,148,151-153).
Others describe the isolation of rifampicin-sensitive enzyme (154-155).
The reason for this discrepancy is not clear. It is possible that a
factor or factors needed for conferring sensitivity to rifampicin were
lost from some enzyme preparations.
In contrast to the above studies several laboratories have reported
that the yeast mitochrondrial RNA polymerases are much larger in size and
resemble more closely the nuclear polymerases. Eccleshall and Criddle
demonstrated three RNA polymerases associated with yeast mitochondria
having molecular weights near 500,000 and showing no sensitivity to
rifamycin (153). However, there was no difference between the nuclear
RNA polymerase I and the mitochondrial RNA polymerase I, suggesting that
their mitochondrial preparation was contaminated with nuclear enzyme.
Reports of multiple RNA polymerases associated with yeast mitochondria
having molecular weights near 500,000 and showing no sensitivity to
rifamycin have come from Rogall and Wintersberger (15) and Benson (156).
Scragg has reported a yeast mitochondrial RNA polymerase with molegular
weight greater than 200,000 that does show a rifamycin sensitive enzyme in
yeast (157). However, despite the low molecular weight (67,000) of the
yeast mitochondrial RNA polymerase polypeptide isolated by Rogall and
Wintersberger (15), this enzyme readily forms aggregates with molecular .