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Stevenson 2000, Dethlefsen and Schmidt 2007). For instance, bacteria that form
visible colonies rapidly (<48 hours) upon exposure to nutritionally complex media
contain an average of 5.5 copies of the SSU rRNA encoding gene, whereas bacteria
that respond slowly contained an average of 1.4 copies (Klappenbach et al. 2000).
These findings reveal phenotypic effects associated with the number of rRNA oper-
ons that underlie the distribution and abundance of bacterial populations in soil.
In addition to determining the number of rRNA operons, we have examined
the efficiency with which the protein-synthesizing machinery operates in microbes.
This machinery typically makes up more than half of a microbe's dry weight and
consumes a majority of the cell's energy during rapid growth. The translation of
mRNA into protein has been studied extensively in model organisms; nevertheless,
while the translational apparatus is qualitatively similar in structure and function
across all known life, little is known about variations between organisms in transla-
tional performance. The macromolecular composition of phylogenetically diverse
oligotrophic and copiotrophic soil bacteria suggests that differences in translational
power (normalized rate of protein synthesis) and associated maintenance costs help
explain the fundamental trade-offs between the rapid growth in copiotrophs vs. the
efficient use of resources in oligotrophs (Dethlefsen and Schmidt 2007). Analysis
of bacterial genomes, in particular the patterns of codon usage in protein-coding
genes, supports this model (Dethlefsen and Schmidt 2005).
When this model is applied to denitrifiers, the positive relationship between the
number of rRNA-coding genes and maximum growth rate becomes apparent (Fig.
6.9). It suggests that the influence of land management on the composition of deni-
trifiers results from disturbance favoring fast-responding (copiotrophic) denitrifiers.
1.0
R
2
= 0.81, p = 0.0009
0.8
0.6
0.4
0.2
0.0
2
4
6
8
rRNA Operon Copy Number
Figure 6.9
. The relationship between growth rate and the number of rRNA operon copies
in sequenced genomes of cultured denitrifying bacteria.
