or very short relaxation times compared with the time constants for the change in environment).
With the central role of the protein synthesizing system (PSS; see Section 2.1.4) in cellular
metabolism, this is often used as a key component in simple structured models. Besides a few
enzymes, the PSS consists of ribosomes (Ingraham
et a l
, 1983), which contain approximately 60%
ribosomal RNA and 40% ribosomal protein. Since the ribosomal RNA makes up more than 80% of
the total stable RNA in the cell (see Note 7.3), the level of the ribosomes is easily identified through
measurements of the RNA concentration in the biomass. As seen in Fig. 7.12, the RNA content of
increases approximately as a linear function of the specific growth rate at steady-state
conditions, and a similar observation is made for other microorganisms (see Fig. 7.12). Thus the
level of the PSS is well correlated with the specific growth rate, and there is no doubt that ATpss (or
XRna) is a good representative of the state of activity of the cell. As seen from the chemostat
experiments in Fig. 7.13, where the RNA concentration is measured in
together with the biomass concentration during the transient from one steady state to another steady
state, the linear relation between jYrma and M
seems to hold throughout the transient.
Most simple structured models are based on a division of the cell into an active and an inactive part,
where the PSS is always included in the active part of the cell (Nielsen and Villadsen, 1992). In
some models the DNA content of the cell, XDNA, is also taken to be part of the active cell
compartment, but even though DNA is an essential cell component, from a mechanistic viewpoint
has virtually nothing to do with the growth rate of cell components (except as a possible
determinant of RNA synthesis rate). This is discussed further in Note 7.4.
Spociflc growth rate (h'1)
Figure 7.12 Measurements of steady-state RNA content in various microorganisms as a function of the
specific growth rate (equal to the dilution rate). The data are compiled from various literature sources.