Regulatory mechanism of septum formation in bacterial cells

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Regulatory mechanism of septum formation in Escherichiacoli was investigated with the emphasis on penicilin-binding protein (PBP) 3, a peptidoglycan-synthesizing enzyme essential for septation. Transcription of cell division/envelope biosynthesis gene cluster including the ftsI gene that encodes PBP 3, an intramolecular domain of PBP3 involved in interaction with other cell division components, physiological function of protease Prc that processes precursor PBP 3 in the C terminus, and the envC gene whose mutation causes irregular cell division/separation were analyzed.1. A transcriptional promoter essential for expression of 9 genes including ftsI was identified at 1.9 kb upstream of ftsI.The genes in the cluster with no known mutant alleles were examined for their null phenotype.2. Dominant negative effect by a mutant PBP 3 with the altered active site is supposed to be due to its interaction with other cell division components. Intragenic suppressor mutations that reversed the effect were isolated and combined with the wild-type active site to give novel PBP 3 mutants that seemed defective in interaction with other division components.3. Cells with defective Prc or producing mature PBP 3 as a primary gene product grew normally. C-terminal Processing seemed nonessential for septation.4. Mutations that suppressed the sensitivity of a Prc-defective strain to osmotic/thermal stresses were not in ftsI.The suppressor gene spr was mapped and cloned.5. The spr mutation caused osmotic/thermal stress sensitivity in the prc^+ background. The gene for PBP 7 was cloned as a multicopy suppressor of the sensitivity and analyzed. Protein Spr was likely to be involved in peptidoglycan metabolism.6. The envC gene was cloned and its nucleotide sequence was determined for wild-type and mutant alleles. Disruption of the chromosomal envC caused irregular cell morphology but not lethality.