comparable to the very high concent

comparable to the very high concentrations (∼3–30 μM) of the
major tRNA species found in rapidly growing E. coli (31).
This concentration scale can be converted to a proteome
fraction (φM) of tRNA-affiliated proteins (referred to henceforth
as T-proteins). This class of proteins includes EF-Tu and the
tRNA synthetases, which are found at constant ratio to EF-Tu in
E. coli (15). Considering the protein mass of EF-Tu and tRNA
synthetases, we obtain a scale of φM ≈ 0.02. This is consistent
with the measured proteome fraction of T-proteins in the cell,
which ranges between 5% and 13% depending on growth conditions
and corresponds to ∼60% of the ribosomal protein mass
(Fig. 1A). As the estimated Michaelis constant corresponds to
the concentration at which translation would proceed at half of
its maximal speed, these numbers indicate that the translation
speed ranges between 70% and 86% of its maximum, consistent
with earlier estimates for Salmonella (16). Our estimate of the
Michaelis constant thus suggests that translation indeed operates
close to the diffusion limit and that macromolecular crowding
imposes a substantial cost to the cell by setting a large scale for
the required allocation of T-proteins.