SUMMARY
Carotenoids are widely used in functional foods, cosmetics, and health
supplements, and their importance and scope of use are continuously
expanding. Here, we characterised carotenoid biosynthetic genes of the
plant-pathogenic bacterium Pantoea ananatis , which carries a
carotenoid biosynthetic gene cluster (including crtE , X ,Y , I , B , and Z ) on a plasmid. Reverse
transcription–polymerase chain reaction (RT-PCR) analysis revealed that
the crtEXYIB gene cluster is transcribed as a single transcript
and crtZ is independently transcribed in the opposite direction.
Using splicing by overlap extension with polymerase chain reaction (SOE
by PCR) based on asymmetric amplification, we reassembledcrtE –B , crtE –B –I , andcrtE –B –I –Y . High-performance liquid
chromatography confirmed that Escherichia coli expressing the
reassembled crtE –B , crtE –B –I , andcrtE –B –I –Y operons produced phytoene,
lycopene, and β-carotene, respectively. We found that the carotenoids
conferred tolerance to UV radiation and toxoflavin. Pantoea
ananatis shares rice environments with the toxoflavin producerBurkholderia glumae and is considered to be the first reported
example of producing and using carotenoids to withstand toxoflavin. We
confirmed that the carotenoid production of P . ananatis is
dependent on RpoS, which is positively regulated by Hfq/ArcZ and
negatively regulated by ClpP, similar to an important regulatory network
of E . coli (HfqArcZ → RpoS Ͱ ClpXP). We
also demonstrated that Hfq-controlled quorum signalling de-represses
EanR to activate RpoS, thereby initiating carotenoid production.
Survival genes such as those responsible for the production of
carotenoids of the plant-pathogenic P . ananatis must be
expressed in a timely manner to overcome stressful environments and
compete with other microorganisms. This mechanism is likely maintained
by a brake with excellent performance, such as EanR.