Fluorescent image of the filamentous cyanobacterium Anabaena PCC 7120. (Image by IHB)

Bacterial cell division, with a few exceptions, is driven by FtsZ through a treadmilling mechanism to remodel and constrict the rigid peptidoglycan (PG) layer. After nitrogen step-down, the filamentous cyanobacterium Anabaena PCC 7120 (Anabaena) is able to form heterocysts, cells that are capable of N₂-fixing but have lost the capacity of division. Previous studies have already showed that heterocyst differentiation depends on the process of cell division, but the genetic mechanism remains unclear.

Recently, the research group led by Prof. ZHANG Chengcai from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences reported that HetF, a known heterocyst-development regulator, functions as a new divisome component. The results were published in mBio.

In Anabaena, hetF is required for the initiation of the differentiation of heterocysts. This study showed that the expression of hetF is active in vegetative cells and down-regulated in heterocysts, suggesting that HetF mainly functions in vegetative cells.

The researchers knocked out hetF in Anabaena using the CRISPR technology, and found that inactivation of hetF impaired cell division under certain conditions. Particularly, under the high-light condition, cells of a ΔhetF mutant stopped dividing which was consistent with increased level of HetF under similar conditions in the wild type.

In the hetF mutant, the rings of FtsZ are still present in the elongated cells, however, PG remodeling activity at the septa, shown by 7-hydroxycoumarin-amino-D-alanine (HADA) labeling, is abolished. This phenotype is similar to that was observed with the inhibition of septal PG synthase FtsI, a divisome protein responsible for septal PG synthesis. In fact, protein-protein interaction assays showed that HetF actually interacts with FtsI directly.

The researchers found that HetF is a membrane protein enriched at midcell and cell-cell junctions. By screening HetF point mutations that do not interact with FtsI, they revealed that HetF is recruited to the divisome by FtsI, and HetF-FtsI interaction is necessary for HetF functioning in cell division.

This study shows that HetF is a member of the divisome, and reveals the distinct features of the cell-division machinery in cyanobacteria.