The csiD promoter (csiD) is essential for the expression of csiD(carbon starvation induced gene), ygaF and the gab genes. The csiD is activated exclusively under carbon starvation conditions and stationary phase during which cAMP accumulates in high concentrations in the cell. The binding of cAMP to the cAMP receptor protein(CRP) causes CRP to bind tightly to a specific DNA site in the csiD promoter, thus activating the transcription of genes downstream of the promoter.
The gabD exerts an additional control over the gabDTP region. The gabD is activated by σ inducing conditions such as hyperosmotic and acidic shifts besides starvation and stationary phase. The gabD promoter on the other hand, is σ dependent and is activated under nitrogen limitation. In nitrogen limiting conditions, the nitrogen regulator Nac binds to a site located just upstream of the promoter expressing the gab genes. The gab genes upon activation produce enzymes that degrade GABA to succinate.