Coordinated regulation of gene expression by E ... - Jacobs University

DISCUSSION
In a study by Balke & Gralla, the changes in supercoiling has been shown to be
dependent on the growth phase rather than on the growth rate [Balke & Gralla, 1987].
In the current work, though there are differences in the growth rates of wild-type, fis
(lower growth rate compared to wild-type) & hns (higher growth rate compared to wildtype)
mutants the supercoiling does not differ much (Figure 12). Since it is known that
in slower-growing cells, transcription from stable RNA promoters is decreased relative
to that in fast-growing cells, the decrease in growth rate in the fis mutant or the increase
in the growth rate of hns mutant can be a direct effect of these proteins on the stable
RNA transcription. The molecular basis of this type of growth control and the role of
FIS will be discussed later. H-NS, the other NAP studied here forms a filament like
structure bridging two DNA duplexes and acting as a zipper [Dame, 2005]. The
increase in negative supercoiling shown here is consistent with the previous reports
[Mojica et al., 1997]. The reason for this increase is not clearly understood. The role derepression
of transcription in hns mutant leading to an increased global supercoiling
(according to twin-domain model of supercoiling distribution) has been ruled out
[Mojica et al., 1997]. Our recent study identified a 10bp AT-rich consensus recognized
by H-NS and serving as a nucleation site to restrain negative supercoiling [Lang et al.,
2007].
FIS protein (1 FIS molecule per 450bp of E.coli genome) is more abundant
than H-NS (1 H-NS dimer per 1400bp) during exponential growth. These two proteins
are known to compete for binding to DNA and their binding also depends on the
supercoiling. FIS is known to preferentially bind DNA with intermediate superhelical
density and it also has been shown to protect DNA of intermediate superhelical density
from relaxation by topoisomerases I [Schneider et al., 1997]. Most of the effect of FIS is
in fine tuning of transcription according to the prevailing situation, as it can both
activate and repress the expression of a certain gene based on the growth phase. An
example for this phenomenon can been seen in a work with osmE promoter by Bordes
et al., [Bordes et al., 2002]. The effect of H-NS on transcription is more straightforward,
it mainly acts as global repressor. In hns mutant 70% of the regulated genes are
associated with Rep group while in fis mutant the Rep genes increased transiently only
during exponential phase (Table 1 & Figure 14). Double-coded genes (Figure 14) from
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