Inactivation of patS produces a Mch phenotype whereas overexpression of patS abolishes differentiation. The primary product of patS is a 17amino acid peptide. The 9-amino acid Niltubacin N-terminal stretch of PatS appears to be involved in processing the peptide, which is needed for immunity against PatS in the differentiating cells in which the peptide is produced. Processing of PatS would render a C-terminal peptide, likely of 8 amino acids, that acting as a morphogen is transferred to the neighboring vegetative cells. PatS appears to interact with HetR and regulate its activity, but the pathway of intercellular transfer of PatS or a peptide derivative of PatS is unknown. However, after prolonged incubation in the absence of combined nitrogen, hetC mutants exhibit a pattern of weakly-fluorescent cells, a characteristic of heterocysts, but, in contrast to heterocysts, they can divide producing a pattern of spaced series of small cells. Because heterocysts are terminal, non-dividing cells, this observation led to the proposal that HetC is involved in the transition to non-dividing cells during heterocyst differentiation. To get clues for the function of HetC and HetP, we have studied the subcellular localization of those two proteins in heterocysts by making use of C-terminal GFP fusion domains expressed from gene constructs present in the cells with copy numbers similar to those of the native genes. In mature heterocysts, HetC-GFP is localized through the heterocyst periphery, and appears especially concentrated near the heterocyst poles. Because different bioinformatics programs predict that HetC has a number of transmembrane segments, it is evident that this protein is targeted to a membrane, and according to Fig. 4C may be targeted to the plasma membrane. In both CSM1 and CSL33, there is more fluorescence near the cell poles. However, it is unclear whether that localization results from the presence of two close membrane units in the heterocyst neck, or whether there is a preferential targeting near the cell poles. In Gramnegative bacteria, genes that encode exporters of toxic peptides are frequently linked to genes that encode a protein that belongs to the membrane-fusion protein family that spans the periplasmic space linking the ABC exporter to an outer membrane channel. Because no homolog of membrane-fusion proteins has been detected in the hetC genomic region, it has been speculated that the HetC substrate could be released to the Anabaena periplasm. It is also possible that HetC acts in connection with a different type of protein.