Evolution of a σ-(c-di-GMP)-anti-σ switch

Area of Science:

• Microbiology
• Molecular Biology
• Evolutionary Biology

Background:

• Filamentous actinobacteria like *Streptomyces* exhibit complex lifecycles involving spore differentiation.
• Cyclic di-GMP (c-di-GMP) regulates this transition by modulating the anti-sigma factor RsiG, which binds the sporulation sigma factor WhiG.

Purpose of the Study:

• To investigate the evolutionary history and structural diversity of RsiG homologs within Actinobacteria.
• To understand the mechanism of c-di-GMP binding and WhiG interaction in both single-motif and twin-motif RsiG variants.

Main Methods:

• Structural analysis of RsiG-c-di-GMP-WhiG complexes from *Streptomyces venezuelae* and *Rubrobacter radiotolerans*.
• Phylogenetic analysis of RsiG homologs across Actinobacteria.
• Biochemical assays to study RsiG function in *R. radiotolerans*.

Main Results:

• RsiG homologs are widespread in Actinobacteria, with variations in c-di-GMP binding motifs.
• Single-motif RsiGs in unicellular bacteria homodimerize to bind c-di-GMP and WhiG, regulating type IV pilus expression.
• Twin-motif RsiGs, arising from gene duplication, function as monomers and are found in other Actinobacteria.

Conclusions:

• The anti-sigma RsiG evolved via intragenic duplication from a homodimeric single-motif protein to a monomeric twin-motif protein.
• This evolutionary trajectory altered RsiG function and regulation across diverse Actinobacteria.
• Structural and functional differences between RsiG variants offer insights into their selective advantages.