Continuous periplasm in a filamentous, heterocyst‐forming cyanobacterium

Summary The cyanobacteria bear a Gram‐negative type of cell wall that includes a peptidoglycan layer and an outer membrane outside of the cytoplasmic membrane. In filamentous cyanobacteria, the outer membrane appears to be continuous along the filament of cells. In the heterocyst‐forming cyanobacteria, two cell types contribute specialized functions for growth: vegetative cells provide reduced carbon to heterocysts, which provide N 2 ‐derived fixed nitrogen to vegetative cells. The promoter of the patS gene, which is active specifically in developing proheterocysts and heterocysts of Anabaena sp. PCC 7120, was used to direct the expression of altered versions of the gfp gene. An engineered green fluorescent protein (GFP) that was exported to the periplasm of the proheterocysts through the twin‐arginine translocation system was observed also in the periphery of neighbouring vegetative cells. However, if the GFP was anchored to the cytoplasmic membrane, it was observed in the periphery of