A Third Class: Functional Gibberellin Biosynthetic Operon in Beta-Proteobacteria.

Area of Science:

• Microbiology
• Plant Science
• Biochemistry

Background:

• Plant-associated microbes, including fungi and bacteria, are known to produce gibberellin A (GA) phytohormones.
• The genes for GA production are encoded in a conserved operon, suggesting horizontal gene transfer between bacterial classes.
• Previous studies identified this operon in alpha-proteobacteria (α-rhizobia) and gamma-proteobacteria.

Purpose of the Study:

• To investigate the presence and function of the GA production operon in beta-proteobacteria (β-rhizobia).
• To biochemically characterize the enzymes involved in GA synthesis from *Paraburkholderia mimosarum*.
• To understand the evolutionary history and functional implications of the GA operon in different bacterial lineages.

Main Methods:

• Biochemical characterization of enzymes encoded by the GA operon from *Paraburkholderia mimosarum* LMG 23256^T.
• Phylogenetic analysis of the GA operon in β-rhizobia compared to other bacterial classes.
• Assessment of enzyme activity, including the identification of gene fusions and missing key enzymes.

Main Results:

• The GA production operon was found to be present and functional in β-rhizobia.
• Enzymes from *P. mimosarum* produced GA₉, a precursor to bioactive GA₄, due to the absence of the cytochrome P450 (CYP115) enzyme.
• Phylogenetic analysis indicated horizontal gene transfer of the operon from gamma-proteobacteria to β-rhizobia, followed by independent loss of CYP115.

Conclusions:

• Beta-proteobacteria harbor a functional GA production operon, acquired via horizontal gene transfer from gamma-proteobacteria.
• The loss of the CYP115 enzyme in β-rhizobia, similar to α-rhizobia, limits their ability to produce bioactive GA₄.
• This suggests potential negative impacts on plant-microbe interactions due to the inability to produce active phytohormones.