GS is subject to completely different regulatory mechanisms in cyanobacteria. Instead of the common NtrC-NtrB two component system, cyanobacteria harbour the transcriptional regulator NtcA which is restricted to this clade and controls expression of GS and a multitude of genes involved in Nitrogen metabolism. Moreover, GS in Cyanobacteria is not covalently modified to raise sensitivity for feedback inhibition. Instead, GS in Cyanobacteria is inhibited by small proteins, termed GS inactivating factors (IFs) whose transcription is negatively regulated by NtcA. These inactivating factors are furthermore regulated by different Non-coding RNAs: The sRNA NsiR4 interacts with the 5'UTR of the mRNA of the GS inactivating factor IF7 ("gifA" mRNA) and reduces its expression. NsiR4 expression is under positive control of the nitrogen control transcription factor NtcA. In addition, expression of the GS inactivating factor IF17 is controlled by a glutamine-binding riboswitch.