Alp7R regulates expression of the actin-like protein Alp7A in Bacillus subtilis

Alp7A is a bacterial actin from Bacillus subtilis plasmid pLS20 that functions in plasmid segregation. Alp7A's function requires that it assemble into filaments that treadmill and exhibit dynamic instability. These dynamic properties require the two other components of the alp7A operon, the downstream alp7R gene and the upstream alp7C sequence, as does the ability of Alp7A to form filaments at its physiological concentration in the cell. Here, we show that these two other components of the operon also determine the amount of Alp7A that is produced in the cell. The deletion of alp7R leads to overproduction of Alp7A, which assembles into large, amorphous, static filaments that disrupt chromosome segregation and cell division. The product of the alp7R gene is a DNA-binding protein that represses transcription of the alp7A operon. Purified Alp7R protein binds specifically to alp7C, which contains two σ(A) promoters embedded within a series of near-repeats of a 10-mer. Alp7R also shows the typical nonspecific binding activity of a DNA-binding protein: Alp7R-GFP (green fluorescent protein) associates with the chromosomes of cells that lack alp7C. When Alp7A-GFP is produced in B. subtilis along with untagged Alp7R, Alp7A-GFP also colocalizes with the chromosome, indicating that Alp7R associates with Alp7A. Hence Alp7R, determines both the activity and the cellular concentration of Alp7A, and it can associate with Alp7A even if it is not bound to alp7C.