Bacteria lack the spindle structure that, in eukaryotic cells, allows the segregation of sister chromosomes to opposite poles. Bacterial sister chromosomes segregate from each other in a process whereby the region containing the origin of replication moves towards the cell pole. Whether these origin regions are anchored at the poles is unknown. However, Richard Losick and colleagues now describe, in Science Express, a protein — RacA (for remodelling and anchoring of the chromosome) — with a polar anchoring, as well as a chromosome remodelling function.

The two sister chromosomes in sporulating Bacillus subtilis condense and form a structure called the axial filament. Near one of the cell poles, a septum is formed that divides the cell into a smaller forespore and a larger mother cell. Losick and co-workers found that racA transcription is switched on during early sporulation and therefore suspected a role for RacA in sporulation. RacA mutants showed delayed septum formation and had a compact DNA mass (nucleoid) in contrast to an extended axial filament in wild-type cells. Also, in ∼50% of the cases examined, the forespore lacked DNA.

Using fluorescence microscopy, Losick and colleagues saw that a RacA–green fluorescent protein (GFP) construct was present at the poles, and a diffuse fluorescence haze indicated that RacA–GFP colocalizes with the nucleoid. RacA–GFP expression was transient and specific for early sporulation, as the fluorescent signal disappeared subsequently.

The authors constructed a strain that allowed inducible expression of RacA and RacA–GFP during growth. Following induction, fluorescent foci were detected at the poles and diffuse fluorescence was visible at the nucleoid. In many cells, the nucleoids had moved towards the poles, which did not occur in uninduced cells. So, RacA localization depends on its own expression, and is sufficient to anchor the chromosomes to the cell poles.

But how is RacA bound to the cell poles? A candidate protein is the cell-division protein DivIVA, which is located at the poles where it sequesters the division inhibitor MinC–MinD (MinCD). As growth in a DivIVA mutant strain is impaired, a MinD DivIVA double-mutant strain was used. RacA–GFP failed to localize to the extreme poles, whereas localization in a MinD mutant was similar to the wild-type strain, which indicates that RacA localization is indeed dependent on DivIVA.

Chromatin immunoprecipitation and subsequent PCR experiments confirmed that RacA colocalizes with the entire nucleoid, and also revealed preferential binding sites for RacA in the replication origin region.

Losick and colleagues have therefore proposed a model in which RacA is a kinetochore-like protein that binds preferentially near the replication origin and anchors the chromosome to the cell pole by binding — directly or indirectly — to DivIVA. In addition, nonspecific binding of RacA throughout the nucleoid allows remodelling of the chromosome into an axial filament structure. A possible role for RacA in polar division requires further investigation.