Requirement of Tyr-550 for cell division. Plasmid pRK2L10 (A and B) and its derivatives carrying the DNA encoding DivL (WT) (C and D), DivLY550F (E and F), and DivLY550H (G and H), respectively, were transformed into strain PC4403 at 24°C. Cells were grown in peptone–yeast extract medium overnight at 24°C, diluted 25-fold in duplicates, and incubated for ca. 12 hr at either 24°C (A, C, E, and G) or 37°C (B, D, F, and H). Samples were photographed under a phase-contrast microscope at ×630 magnification (23).