Modeling of Growth Kinetics
267
Figure 7.13
Measurements of the RNA content of
L, cremoris
and of the specific growth rate in a
glucose-limited chemostat. The chemostat is at steady state at the dilution rate of 0.075 h'1
during the first
4.3 h of monitoring, whereafter
D
is changed to 0.409 h 1. I1 is calculated from the biomass concentration
profile (not shown), and it is noted that the l1 profile closely mimics the RNA profile with an overshoot
before it settles down at the new
D
value. The glucose concentration profile is also shown. At
D
= 0.075 h"1,
s
= 5.5 mg L 1
and after a large excursion (to 0.75 g L ') the glucose concentration reaches a new
steady-state value of 10 mg L 1. Note the dramatic dependence of the RNA content of cells on glucose
concentration in the low concentration range
s
~ 5-10 mg L 1. The data are taken from Benthin
et al.
(1991).
Note 7.3 Stable and unstable RNA
In microbial cells there are three types of RNA:
1.
Ribosomal RNA (rRNA)
2.
Transfer RNA (tRNA)
3.
Messenger RNA (mRNA)
Both rRNA and tRNA are fairly stable, whereas mRNA has a typical half-life of 1-2 min [1.3 min is an
average value for
E, coli
(Ingraham
et at.,
1983)]. For an
E. coli
cell at a specific growth rate of 1.04 h'*
1
2
3
the
relative content of the three types of RNA is 5% mRNA, 18% tRNA, and 77% rRNA (see Table 2.5), and
the relative content of stable RNA (tRNA plus rRNA) is therefore 95%. With the extremely low half-life of
mRNA, this will normally not be included in the RNA measured by standard techniques [see, e.g., Benthin
et al.
(1991)], and the data specified for RNA measurements therefore represents the sum of tRNA and
rRNA. With the values specified above, it is found that rRNA accounts for approximately 80% of the total
measured RNA, and measurements of RNA can therefore be considered a good estimate for the ribosome
level in the cell.
Whereas the content of stable RNA increases with the specific growth rate (see Fig. 7.12), the tRNA content
decreases with the specific growth rate. Thus the relative content of tRNA is higher than 18% at specific
growth rates below 1.04 h'1. However, tRNA comprise a small fraction of the total RNA and even at very
low specific growth rates it is less than 25% of the total RNA (Ingraham
et al.,
1983). It is therefore still
reasonable to consider the stable RNA content of the cell to be proportional with the ribosome level._______
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