(C) Addition of aztreonam to a culture of strain LT2 in E medium induces filaments identical to those formed by a His c strain in high-glucose medium (compare panels A and C). (B) An isogenic His + strain (LT2) grown under the same conditions does not form filaments. (A) Filaments formed by the His c strain TR6753 grown in E medium containing 2% glucose. Microscopic photographs of cells and filaments prepared with Hiraga’s fluo-phase combined method ( 15). typhimurium in the presence of 2% glucose give rise to colonies which are distinctly wrinkled ( 9, 19, 25). A difference is that ftsI and ftsA mutants are conditional (thermosensitive) lethals unable to form colonies under restrictive conditions ( 3, 18), while the filaments produced by His c strains of S. FtsA is a membrane-bound protein that interacts with PBP3 ( 22, 31).
The ftsI gene encodes PBP3, an essential cell division protein involved in septum formation ( 27). The filaments are similar in morphology and length to those formed by ftsI and ftsA mutants of E. Moreover, the presence of blunt constrictions indicates that the division block lies beyond the stage of FtsZ action ( 8). The His c strain formed long filaments which contained evenly spaced nucleoids, indicating that their division defect is unrelated to DNA synthesis or chromosome partition (Fig. Nucleoid staining was achieved with DAPI (4′,6-diamino-2-phenylindole). Mid-exponential-phase cultures of strains LT2 ( hisO +) and TR6753 ( hisO1242 ) were observed under the microscope by using Hiraga’s fluo-phase combined method, a procedure that permits the simultaneous observation of nucleoids and cells ( 15). The latter observation suggests that HisHF overproduction may cause a shortage in PBP3 substrate. These contradictory data are tentatively reconciled by the ability of d-cycloserine to suppress filamentation in His c mutants. We also describe the unexpected finding that strains that overproduce IGP synthase contain wild-type levels of active penicillin-binding protein 3 (PBP3). typhimurium His c strains is a block in septum formation, as proposed by Frandsen and D’Ari ( 11). We show below that the cell division defect of S. coli, the cell division defect of His c strains is unrelated to the SOS response and does not involve the cell division inhibitor SulA ( 11, 12).
The involvement of AICAR has been also ruled out ( 10, 11). However, division inhibition does not require metabolic flow through the histidine biosynthetic pathway, suggesting that HisH and HisF trigger filamentation through an activity unrelated to IGP synthesis ( 10, 19). HisH and HisF are subunits of the heterodimeric imidazole-glycerol-phosphate synthase ( 1, 34), which catalyzes the formation of imidazole-glycerol-phosphate (IGP) with release of the purine precursor AICAR (5-aminoimidazole-4-carboxamide ribonucleotide) ( 17, 26). A similar response has been described for Escherichia coli ( 11). Wrinkledness reflects cell filamentation ( 12, 19), which is triggered by overproduction of hisH and hisF gene products ( 5, 9, 19). When histidine-constitutive (His c) mutants of Salmonella typhimurium were first isolated, the authors noted that high levels of histidine biosynthetic enzymes caused wrinkled colony morphology on 2% glucose plates ( 25).
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