Isolation and characterization of a unique division mutant of Bacillus megaterium

SUMMARY: A filamentous division mutant, PV302, of Bacillus megaterium QM B1551 was isolated while screening for sporulation-defective mutants after nitrosoguanidine mutagenesis. Both phase-contrast and electron microscopy revealed that the mutant produced small spherical cells as well as filaments. It also accumulated large amounts of poly-β-hydroxybutyrate. Poly-β-hydroxybutyrate accounted for 16% of the dry weight of the mutant strain even after 28 h growth. In comparison to the parental strain, the division mutant also showed both an inability to sporulate and a reduced growth rate. All these phenotypes transduced together. Revertants gained the ability to sporulate, divide, and grow normally. Transductional mapping of the mutation, designated div-1, established a new linkage group for B. megaterium consisting of div-1 and the pyrimidine biosynthesis genes pyrDBCF. The spherical cells were separated from filaments by sucrose gradients and were tested for nucleic acid content and viability. The purified spherical cell fraction contained one-fifth the amount of DNA per mg protein as compared with the filamentous cell fraction and was shown to contain both non-viable minicells and some cells capable of growing after a lag of about 4 h. This suggests that the mutation not only causes defects in septum placement and sporulation, but may possibly affect DNA partitioning.