Under nitrogen-limiting conditions, the multicellular and diazotrophic cyanobacterium Nostoc PCC 7120 differentiate ∼10% of its cells to become specialized nitrogen-fixing heterocysts. The decision to commit into terminal differentiated cells occurs ∼13 h after the induction of differentiation. While the initiation of this process is rather well understood, the mechanisms that govern the transition to final commitment and to maturation are still unknown. In the Fun-Date project, we aim at addressing this point by elucidating the interplay between an ABC-transporter (HetC), HetP and a transcriptional regulator (PatB). Our hypothesis is that the transporter HetC modulates the activity of PatB, and therefore gene expression, through the transport of HetP which otherwise inhibits PatB. Genetic and biochemical approaches will be used to analyze the functional interactions between the three proteins, highlighting the role of this network to achieve terminal differentiation.