DISCUSSION
This study was undertaken with the intentions of examining the
mechanism by which Euglena gracilis, a eukaryotic cell, controls mito
chondrial development. Since the DNA dependent RNA polymerase has been
implicated in controlling development in prokaryotic systems it was felt
that the mitochondrial RNA polymerase could be a major factor controlling
development of the organelle. Therefore, the mitochondrial RNA synthesiz
ing activities were studied with a view toward elucidating the components
of the transcriptional process, in particular characterizing the mitochon
drial DNA dependent RNA polymerase. The RNA synthesizing activity was
first studied in isolated mitochondria because it was felt that it was
necessary to characterize such general properties as sensitivity to in
hibitors, types of RNA being synthesized, precursor requirements, metallic
ion requirements, and temperature optimum. The streptomycin bleached
aplastidic mutant was used for these studies because it provided a means
by which processes controlling mitochondrial development could be distin
guished from those controlling chloroplast development.
Mitochondria were isolated by a procedure that selected for the most
enzymatically active organelles (33) based on succinic dehydrogenase
activity. This procedure also provided purified mitochondria that con
tained mostly mitochondrial DNA (70%) based on the buoyant densities in
CsCl (Figure 2). Although some contaminating nuclear DNA was in the
preparation it represented less than 1% of the total nuclar DNA originally
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