Member of the two-component regulatory system GacA/GacS which controls the expression of secondary metabolites and extracellular products. Activates GacA by phosphorylation (Probable). GacA acts (probably primarily) by activating expression of CsrA1 and CsrA2 antagonist small RNAs (sRNA) RsmX, RsmY and RsmZ which bind to and prevent translation repression by CsrA1 and CsrA2 (PubMed:11807065, PubMed:14622422, PubMed:15601712, PubMed:16286659). Involved in the regulation of secondary metabolism and in the synthesis of the antifungal factors cyanide, 2,4-diacetylphloroglucinol and pyoluteorin (PubMed:15601712). Exercises positive post-transcriptional control over the hcnABC and aprA genes; acts upstream of CsrA2 (rsmA) (PubMed:10570200). Controls expression of CsrA1 and CsrA2 antagonist sRNAs RsmX, RsmY and probably RsmZ (PubMed:11807065, PubMed:14622422, PubMed:15601712, PubMed:16286659). Probably controls expression of csrA1 (rsmE) and csrA2 (PubMed:15601712). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Activation requires a sequential transfer of a phosphate group from a His in the primary transmitter domain, to an Asp in the receiver domain and to a His in the secondary transmitter domain. Greatly decreased expression of aprA, hcnA and phlA (PubMed:10570200, PubMed:11807065, PubMed:14622422, PubMed:15601712). Partially suppressed by a deletion of csrA2 (rsmA) (PubMed:10570200, PubMed:15601712). Fully suppressed by deletion of csrA1 (rsmE) and csrA2 (PubMed:15601712). Its deletion is also suppressed by overexpression of small RNAs (sRNA) RsmZ and RsmY (PubMed:11807065, PubMed:14622422). Loss of expression of sRNA RsmY (PubMed:14622422). Loss of protection of cucumber plants from fungal infection (PubMed:14622422, PubMed:16286659).