Septal localization of the SpoIIIE chromosome partitioning protein in Bacillus subtilis

The 787 amino acid SpoIIIE protein of Bacillus subtilis is required for chromosome partitioning during sporulation. This process differs from vegetative chromosome partitioning in that it occurs after formation of the septum, apparently by transfer of the chromosome through the nascent septum in a manner reminiscent of plasmid conjugation. Here we show that SpoIIIE is associated with the cell membrane, with its soluble C-terminal domain located inside the cell. Immunofluorescence microscopy using affinity-purified anti-SpoIIIE antibodies shows that SpoIIIE is targeted near the centre of the asymmetric septum, in support of a direct role for SpoIIIE in transport of DNA through the septum. We also report on the isolation of a mutation affecting the N-terminal hydrophobic domain of SpoIIIE that interferes with targeting to the septum and blocks DNA transfer. This mutation also causes de-localization of the activity of the normally prespore-specific sigma factor, sigmaF, consistent with the notion that SpoIIIE can form a seal between the chromosomal DNA and the leading edge of the division septum.