GacRsm

GacRsm is a regulatory RNA described as a central component of the Gac/Rsm signaling axis in several Gram-negative bacteria. It is transcribed in response to the GacS/GacA two-component system and acts to modulate post-transcriptional gene expression by titrating RNA-binding proteins of the RsmA/CsrA family. Through this interaction, GacRsm indirectly controls the translation and stability of a broad set of mRNAs involved in virulence, secondary metabolism, motility, biofilm formation, and stress responses.

Molecular mechanism: GacRsm functions as a non-coding RNA containing multiple conserved sequence motifs, including GGA-containing loop structures that are recognized by RsmA-family proteins. Binding of RsmA by GacRsm sequesters the repressor from target mRNAs, allowing increased translation or stability. The mechanism mirrors the mode of action of other Gac-regulated sRNAs (e.g., RsmY/Z in Pseudomonas), but GacRsm is distinguished by its regulatory scope and its own transcriptional regulation.

Regulation and impact: Expression of GacRsm is controlled by environmental cues sensed by GacS and can be modulated by global regulators and sigma factors. In many pathogens and plant-associated bacteria, GacRsm activity promotes traits linked to survival in hostile environments and host interaction, such as pigment production, secretion systems, and biofilm formation. Loss of GacRsm function often reduces virulence or affects surface-associated behaviors.

Distribution and significance: GacRsm homologs or functionally equivalent RNAs have been reported in several genera, most prominently in Pseudomonas species. Because of its central role, GacRsm is a frequent subject of studies on post-transcriptional regulation and host-microbe interactions.