19.7.2 Factor b: nifA
DNA sequence analysis indicates that some bacterial strains do not encode an acdR
gene but are nevertheless still able to produce active enzymes. This observation led
to the discovery of another possible mechanism of transcriptional regulation of this
gene in some bacteria. For instance, in some strains of Rhizobia and Mesorhizobium
the acdS gene is transcriptionally regulated under the control of a nifA promoter (the
major function of this promoter is to control the transcription of all nitrogen fixation
(nif) genes) (Nukui et al. 2006). In this case, the nifA promoter consists of nifA1 and
nifA2 promoters that are located upstream of the acdS gene with a σ54 RNA
polymerase recognition site. The proposed mechanism of regulation of acdS by
nifA has been suggested to come from the nifA2 promoter that interacts with σ54
RNA polymerase and favors acdS transcription (Singh et al. 2015). The precise role
of nifA1 in the expression of acdS is still an open question (Nukui et al. 2006). This
mode of regulation may benefit the nodules that contain these Rhizobia and
Mesorhizobium strains in preventing their premature senescence that is normally
caused by excessive ethylene levels.
19.7.3 Factor c: RpoS
The sigma factor, RpoS, mainly regulates the genes that are expressed when bacteria
enter the stationary phase of their growth or genes that are expressed in response to a
number of stress stimuli (Hengge-Aronis 2002). In this way, RpoS is considered to
be the main stress modulator in β and γ Proteobacteria (Osiriphun et al. 2009). Since
ACC deaminase is mainly produced in the stationary phase of growth when bacteria
face stress from the environment (Saleh and Glick 2001), the relationship between
the expression of acdS and rpoS was investigated (Shah et al. 1998). In this study, it
was found that the overexpression of rpoS affected two closely related bacterial
strains differently. Enterobacter cloacae CAL2, an ACC deaminase positive strain,
was genetically transformed with multiple copies of rpoS gene on a plasmid. The
resulting overexpression of rpoS gene increased the ACC deaminase level by
approximately 30% (Saleh and Glick 2001). On the other hand, when the same
approach was used in bacterial strain Pseudomonas sp. UW4, the ACC deaminase
levels decreased by 20% compared with the untransformed wild type (Saleh and
Glick 2001). It is interesting to note that the acdS genes in these two bacteria exhibit
96% identity but are apparently controlled by different transcriptional regulators
(Grichko and Glick 2001; Shah et al. 1998).
384
S. Ali and B. R. Glick
DNA sequence analysis indicates that some bacterial strains do not encode an acdR
gene but are nevertheless still able to produce active enzymes. This observation led
to the discovery of another possible mechanism of transcriptional regulation of this
gene in some bacteria. For instance, in some strains of Rhizobia and Mesorhizobium
the acdS gene is transcriptionally regulated under the control of a nifA promoter (the
major function of this promoter is to control the transcription of all nitrogen fixation
(nif) genes) (Nukui et al. 2006). In this case, the nifA promoter consists of nifA1 and
nifA2 promoters that are located upstream of the acdS gene with a σ54 RNA
polymerase recognition site. The proposed mechanism of regulation of acdS by
nifA has been suggested to come from the nifA2 promoter that interacts with σ54
RNA polymerase and favors acdS transcription (Singh et al. 2015). The precise role
of nifA1 in the expression of acdS is still an open question (Nukui et al. 2006). This
mode of regulation may benefit the nodules that contain these Rhizobia and
Mesorhizobium strains in preventing their premature senescence that is normally
caused by excessive ethylene levels.
19.7.3 Factor c: RpoS
The sigma factor, RpoS, mainly regulates the genes that are expressed when bacteria
enter the stationary phase of their growth or genes that are expressed in response to a
number of stress stimuli (Hengge-Aronis 2002). In this way, RpoS is considered to
be the main stress modulator in β and γ Proteobacteria (Osiriphun et al. 2009). Since
ACC deaminase is mainly produced in the stationary phase of growth when bacteria
face stress from the environment (Saleh and Glick 2001), the relationship between
the expression of acdS and rpoS was investigated (Shah et al. 1998). In this study, it
was found that the overexpression of rpoS affected two closely related bacterial
strains differently. Enterobacter cloacae CAL2, an ACC deaminase positive strain,
was genetically transformed with multiple copies of rpoS gene on a plasmid. The
resulting overexpression of rpoS gene increased the ACC deaminase level by
approximately 30% (Saleh and Glick 2001). On the other hand, when the same
approach was used in bacterial strain Pseudomonas sp. UW4, the ACC deaminase
levels decreased by 20% compared with the untransformed wild type (Saleh and
Glick 2001). It is interesting to note that the acdS genes in these two bacteria exhibit
96% identity but are apparently controlled by different transcriptional regulators
(Grichko and Glick 2001; Shah et al. 1998).
384
S. Ali and B. R. Glick