Sibling Paenibacillus dendritiformis bacterial colonies grown on
low-nutrient agar medium mutually inhibit growth through secretion
of a lethal factor. Analysis of secretions reveals the presence of
subtilisin (a protease) and a 12 kDa protein, termed sibling lethal
factor (Slf). Purified subtilisin promotes the growth and expansion
of P. dendritiformis colonies, whereas Slf is lethal and lyses
P. dendritiformis cells in culture. Slf is encoded by a gene belonging
to a large family of bacterial genes of unknown function, and the
gene is predicted to encode a protein of approximately 20 kDa,
termed dendritiformis sibling bacteriocin. The 20 kDa recombinant
protein was produced and found to be inactive, but exposure to
subtilisin resulted in cleavage to the active, 12 kDa form. The experimental
results, combined with mathematical modeling, show
that subtilisin serves to regulate growth of the colony. Below a
threshold concentration, subtilisin promotes colony growth and
expansion. However, once it exceeds a threshold, as occurs at the
interface between competing colonies, Slf is then secreted into the
medium to rapidly reduce cell density by lysis of the bacterial cells.
The presence of genes encoding homologs of dendritiformis sibling
bacteriocin in other bacterial species suggests that this mechanism
for self-regulation of colony growth might not be limited
to P. dendritiformis.