Channels that cross cell walls and connect the cytoplasm of neighboring cells in multicellular cyanobacteria are pivotal for intercellular communication. We find that the product of the gene all1140 of the filamentous cyanobacterium Anabaena sp. PCC 7120 is required for proper channel formation. All1140 encodes an amidase that hydrolyses purified peptidoglycans. An All1140-GFP fusion protein is located at the Z-ring in the periplasmic space during most of the cell cycle. An all1140-null mutant (M40) was unable to grow diazotrophically, and no mature heterocysts were observed in the absence of combined nitrogen. Expression of two key genes, hetR and patS, was studied in M40 using GFP as a reporter. Upon nitrogen step-down, the patterned distribution of green fluorescent cells in filaments seen in the wild type were not observed in mutant M40. Intercellular communication in M40 was studied by measuring fluorescence recovery after photobleaching (FRAP). Movement of calcein (622 Da) was aborted in M40, suggesting that the channels connecting the cytoplasm of neighboring cells are impaired in the mutant. The channels were examined with electron tomography; their diameters were nearly identical, 12.7 nm for the wild type and 12.4 nm for M40, suggesting that AmiC3 is not required for channel formation. However, when the cell wall sacculi isolated by boiling were examined by EM, the average sizes of the channels of the wild type and M40 were 20 nm and 12 nm, respectively, suggesting that the channel walls of the wild type are expandable and that this expandability requires AmiC3.
Keywords: channels; cyanobacteria; intercellular communication; peptidoglycan.