In a growing B. subtilis cell, the tubulin-like protein FtsZ assembles into a ring structure at the division plane in the middle of the cell. Upon sporulation, cell division is asymmetric, and FtsZ rings form at the poles. Previous models suggested that the shift occurred when FtsZ assembly was blocked at the midcell and activated at the poles.
Ben-Yehuda and Losick examined a GFP fusion of FtsZ during sporulation and discovered that this model was inadequate. The fusions revealed a spiral-like intermediate of FtsZ, which over time extended from the midcell toward both poles. The spirals eventually gave way to polar rings, one of which became the division plane. Thus, generation of polarity requires that the ring still form at the midcell, with the spirals causing its relocation. The spirals were also seen moving from the poles to the midcell when bacteria entering sporulation were returned to growth medium.
The spirals may result from elevated levels of FtsZ present during sporulation, as overexpression of ftsZ in Escherichia coli has also been shown to cause the formation of spirals. “Possibly, it is an intrinsic property of FtsZ that high concentrations cause it to form spirals,” says Losick. “Or maybe the ring structure is actually a tight spiral, and only the periodicity changes.” ▪