Two Functionally Deviating Type 6 Secretion Systems Occur in the Nitrogen-Fixing Endophyte Azoarcus olearius BH72

Area of Science:

• Microbiology
• Bacterial Physiology
• Plant-Bacterial Interactions

Background:

• Type VI protein secretion systems (T6SSs) are crucial for bacterial interactions but remain poorly understood in many plant-associated microbes.
• Azoarcus olearius is a nitrogen-fixing grass endophyte with two putative T6SS gene clusters, tss-1 and tss-2.

Purpose of the Study:

• To functionally dissect the two T6SSs in Azoarcus olearius strain BH72.
• To differentiate the roles and regulation of T6SS-1 and T6SS-2 in this model endophyte.

Main Methods:

• Comparative analysis of wild-type and T6SS mutant strains (tss-1 and tss-2).
• Assessing Hcp secretion as an indicator of T6SS activity.
• Investigating post-translational regulation mechanisms, including phosphorylation.

Main Results:

• Only T6SS-2 demonstrated activity, indicated by Hcp secretion under both aerobic and nitrogen-fixing conditions.
• T6SS-2 activity is regulated by phosphorylation (TagE/TagG) and TagF, similar to Pseudomonas aeruginosa.
• TSS-1 genes are upregulated during nitrogen fixation, but T6SS-1 itself was not active, suggesting unknown post-translational regulation.

Conclusions:

• Azoarcus olearius possesses two distinct T6SSs with specialized functions.
• T6SS-2 is a functional secretion system regulated by known phosphorylation pathways.
• TSS-1's role and regulation remain to be elucidated, but it appears linked to the nitrogen-fixing lifestyle.