19.7.4 Factor d:
Other Mechanisms
Many ACC deaminase positive bacteria possess acdR or similar genes in relatively
close proximity to acdS (about 50 to a few hundred base pairs upstream of the acdS
gene). However, some ACC deaminase positive bacteria have completely or partially lost the acdR gene but are nevertheless still able to produce active enzyme. In
other cases, acdR or a similar LRP producing gene is located as far as 9 kb from the
acdS gene, but is still able to control acdS regulation (Nascimento et al. 2014). In
two strains from the genus Burkholderia, namely Burkholderia sp. CCGE1002 and
Burkholderia phymatum STM815, no acdR gene has been detected, instead, these
bacterial strains contain two copies of the acdS gene; one is on the bacterial
chromosome and the other copy is located on the mega plasmid that these bacteria
contain (Singh et al. 2015). In another member of the same genus, Burkholderia
xenovorans LB4000, the LysR family of transcription regulatory elements has been
found to be in close proximity to the acdS gene (Singh et al. 2015). Some members
of Acinetobacter spp. and Proteobacteria spp. (e.g., Brenneria sp. EniD312,
Dickeya spp., and Pantoea sp. At-9B) are also thought to be under the same
(LysR) transcriptional regulation (Nascimento et al. 2014). Moreover, in many
members of Actinobacteria and Meiothermus, another gene, responsible for the
transcription of regulatory protein GntR, has been found in the neighborhood of
the acdS gene and has been suggested to play a role in the regulation of the acdS
gene in these bacteria (Nascimento et al. 2014). In Saccharopolyspora erythraea
NRRL 233 and Streptomyces hygroscopicus ATCC 53653, the acdS gene is apparently a part of the MFS (major facilitator superfamily) proteins and may be regulated
by the same operon regulatory elements, prominently the M20 peptidase
(Nascimento et al. 2014). From the aforementioned, it is clear that the regulation
of the acdS gene is a rather complex process with different microbes utilizing
somewhat different strategies.
19.8 Future Prospects
While PGPB utilize a wide range of mechanisms to facilitate plant growth (Glick
2012), especially in the presence of many different biotic and abiotic environmental
stresses, the use of the enzyme ACC deaminase to lower plant ethylene levels is
probably one of the most important of those mechanisms. If the use of PGPB is to
become a future cornerstone of a new paradigm in agricultural practice where the use
of harmful chemicals is severely constrained, it is essential that scientists develop a
detailed fundamental understanding of precisely how PGPB function. This understanding should enable scientists to more reproducibly employ the use of PGPB in
horticulture, silviculture, and environmental clean-up as well as agriculture (Reed
and Glick 2013).
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
385
Other Mechanisms
Many ACC deaminase positive bacteria possess acdR or similar genes in relatively
close proximity to acdS (about 50 to a few hundred base pairs upstream of the acdS
gene). However, some ACC deaminase positive bacteria have completely or partially lost the acdR gene but are nevertheless still able to produce active enzyme. In
other cases, acdR or a similar LRP producing gene is located as far as 9 kb from the
acdS gene, but is still able to control acdS regulation (Nascimento et al. 2014). In
two strains from the genus Burkholderia, namely Burkholderia sp. CCGE1002 and
Burkholderia phymatum STM815, no acdR gene has been detected, instead, these
bacterial strains contain two copies of the acdS gene; one is on the bacterial
chromosome and the other copy is located on the mega plasmid that these bacteria
contain (Singh et al. 2015). In another member of the same genus, Burkholderia
xenovorans LB4000, the LysR family of transcription regulatory elements has been
found to be in close proximity to the acdS gene (Singh et al. 2015). Some members
of Acinetobacter spp. and Proteobacteria spp. (e.g., Brenneria sp. EniD312,
Dickeya spp., and Pantoea sp. At-9B) are also thought to be under the same
(LysR) transcriptional regulation (Nascimento et al. 2014). Moreover, in many
members of Actinobacteria and Meiothermus, another gene, responsible for the
transcription of regulatory protein GntR, has been found in the neighborhood of
the acdS gene and has been suggested to play a role in the regulation of the acdS
gene in these bacteria (Nascimento et al. 2014). In Saccharopolyspora erythraea
NRRL 233 and Streptomyces hygroscopicus ATCC 53653, the acdS gene is apparently a part of the MFS (major facilitator superfamily) proteins and may be regulated
by the same operon regulatory elements, prominently the M20 peptidase
(Nascimento et al. 2014). From the aforementioned, it is clear that the regulation
of the acdS gene is a rather complex process with different microbes utilizing
somewhat different strategies.
19.8 Future Prospects
While PGPB utilize a wide range of mechanisms to facilitate plant growth (Glick
2012), especially in the presence of many different biotic and abiotic environmental
stresses, the use of the enzyme ACC deaminase to lower plant ethylene levels is
probably one of the most important of those mechanisms. If the use of PGPB is to
become a future cornerstone of a new paradigm in agricultural practice where the use
of harmful chemicals is severely constrained, it is essential that scientists develop a
detailed fundamental understanding of precisely how PGPB function. This understanding should enable scientists to more reproducibly employ the use of PGPB in
horticulture, silviculture, and environmental clean-up as well as agriculture (Reed
and Glick 2013).
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
385