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12.5. Conclusion and Discussion
Fig. 12.3a shows the time courses of the average levels of 14 each of the glycolytic
and respiratory mRNA molecules during the 850 minutes of observation after
shifting glucose to galactose. The time course of the average transcription rates of
the same sets of glycolytic and respiratory genes are displayed in Fig. 12.3b. The
most striking feature of these figures is that, despite the similarity between the
time courses of the transcription rates (
TR
) of glycolytic and respiratory genes,
those of the corresponding transcript levels (
TL
) are quite different. In fact, the
TL
trajectories of glycolytic and respiratory genes change in opposite directions dur-
ing the period between 5 and 360 minutes after glucose-galactose shift, whereas
the corresponding
TR
trajectories almost coincide. As shown in Fig. 12.4, these
opposite changes in
TL
appear to be the consequences of the opposite changes in
the degradation rates of glycolytic and respiratory mRNA molecules.
The qualitative features of the temporal behaviors of
TL
and
TR
changes dis-
played in Fig. 12.3a and 12.3b are summarized in Table 12.2. As indicated in the
first two rows, the total observational period of 850 minutes are broken down to
5 phases, labeled
I
through
V
. During Phase I, the transcript levels of both gly-
colytic and respiratory genes decrease precipitously although the corresponding
transcription rates increase, most likely because the stress induced by glucose-
galactose shift increase transcript degradation rates more than can be compensated
for by increased transcription. This interpretation is supported by the transcrip-
tion/degradation ratio of 0.5 calculated for Phase I (see Fig. 12.4). During Phases
II and III, the glycolytic transcript levels decrease by 2 fold, whereas the respi-
ratory transcript levels increase by 4 fold. Since the corresponding transcription
rates of both the glycolytic and respiratory genes decline rapidly followed by a
plateau, the increased respiratory mRNA levels cannot be accounted for in terms
of transcriptional control but must implicate degradational control. That is, just
as the removal of glucose “de-induces” glycolytic mRNA molecules (leading to
the declining
TL
and
TR
trajectories for glycolysis in Fig. 12.3a and 12.3b), so it
might repress the degradation of respiratory mRNA molecules, leading to a rise in
respiratory mRNA levels as seen in Fig. 12.3a between the second and fourth time
points. This phenomenon may be referred to as “glucose de-induction” in analogy
to glucose induction [7, 18, 20]. If this interpretation is correct, one intriguing hy-
pothesis suggests itself that glucose normally keeps the respiratory mRNA levels
low by both enhancing the degradation and repressing the synthesis of respiratory
mRNA molecules. During Phase IV, both
TL
and
TR
for glycolytic and respira-
tory genes increase, and this may be attributed to galactose induction [11, 18, 34].
In support of this interpretation, it was found that glucose-galactose shift induced
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