MucR/Ros family in plant-associated bacteria: from transcriptional regulators to c-di-GMP effectors

Abstract

The MucR/Ros family are DNA-interacting proteins and pleiotropic regulators of key aspects of the lifecycle in α-proteobacteria, particularly in plant-associated bacteria. The early finding that MucR/Ros proteins contain a C-terminal domain resembling the Cys2His2 eukaryotic Zn-finger domain, indicated MucR/Ros were classical transcription factors binding specific DNA sequences. However, the pleiotropic phenotypes displayed by ros/mucR mutants and the involvement in regulating diverse functions, including cell cycle progression, symbiosis, virulence, motility, production of cell surface components, pointed MucR/Ros as global regulators. Structural and functional studies have positioned MucR/Ros as novel H-NS (Histone-like Nucleoid Structuring)-like proteins in α-proteobacteria, that bind AT-rich DNA sequences and are able of bridging DNA through formation of higher order oligomers. MucR/Ros have also been positioned as xenogeneic silencers (XS), with key roles in horizontal gene acquisition and genome evolution. At least one MucR member is reported to bind the bacterial universal second messenger c-di-GMP, what could thus represent an anti-silencing mechanism as proposed for other bacterial XS. The co-existence of functionally and structurally heterogeneous MucR homologs in the same genome adds further complexity but also warrants renovated interest to decipher the multiple roles of MucR in this important group of bacteria.