Pseudomonad GacS/GacA lifestyle-control phosphorelay

The pseudomonad Gac two-component system, a global lifestyle switch between motile/acute and sessile/community behaviour. The membrane hybrid sensor kinase GacS autophosphorylates and relays phosphate through its own receiver and HPt modules to the conserved receiver aspartate of the LuxR-type response regulator GacA, which then acts as a DNA-binding transcriptional regulator of surface-associated behaviours including adhesin expression and biofilm development. A second membrane hybrid sensor kinase, RetS, acts on the same relay as an opposing input rather than as an independent pathway. The module covers sensing, His->Asp phosphotransfer, and the transcriptional output; the individual downstream regulons (type VI secretion, exopolysaccharide synthesis, c-di-GMP effectors) are separate modules.

The module is scoped to Pseudomonas because the GacS/GacA/RetS arrangement and its lifestyle output are characterised in that genus; the underlying relay chemistry is the taxon-neutral MODULE:two_component_relay motif, which the inner phosphorelay bundle declares. GacS and RetS are hybrid kinases whose HPt module is a domain of the sensor protein rather than a discrete histidine-phosphotransfer protein, so the bundle collapses the template's three protein tiers into two, recorded as a deviation. The KT2440 outputs that are experimentally supported gene-by-gene (adhesin/biofilm control for GacA, K1-T6SS repression for RetS) are recorded in the gene reviews; only the adhesin/biofilm arm is modelled here as the transcriptional output, because it is the arm with direct transcriptional evidence for the regulator itself. RsmA/RsmE and the CrcZ/CrcY-class small RNAs that carry the Gac signal post-transcriptionally in other pseudomonads are deliberately outside this boundary: they have no reviewed gene grounding in this repository yet.