Visualizing multiple constrictions in spheroidal Escherichia coli cells

An Escherichia coli cell grows by elongation and divides in a perpendicular plane. Alternating planes of successive divisions in three dimensions can only be ascertained when multiple constrictions exist simultaneously in lar ge, spheroidal cells (with extended constriction process), if the division signals are enhanced. Large, spheroidal cells are obtained by a brief mecil linam treatment, and more frequent divisions are achieved by manipulating t he rate of chromosome replication without affecting cell mass growth rate. Such a procedure has recently been performed by thymine-limitation of E. co li K12 strain CR34 (Zaritsky et al., Microbiology 145 (1999), 1052-1022). E nhancing the replication rate in cells with multi-forked replicating chromo somes (by addition of deoxyguanosine) shortens the intervals between succes sive terminations and thus triggers divisions more frequently. Monoclonal a ntibodies against FtsZ were used to visualize the rings of secondary constr ictions, but apparent shortage of FtsZ to complete rings over wide cells al lowed assembly of arcs only. The arcs observed were not parallel nor perpen dicular; the tilted constriction planes are consistent with our 3-D 'nucleo id segregation' model for division under conditions which relieve the cylin drical constraint for nucleoid segregation by the bacillari peptidoglycan s acculus (Woldringh et al., J. Bacteriol. 176 (1994) 6030-6038). The shortag e in FtsZ may explain the longer time required to complete the division pro cess in wide cells with long circumferences, observed during thymine step-u p. Overexpression of fusion protein FtsZ-GFP on a multi-copy plasmid should circumvent the shortage. (C) 1999 Societe francaise de biochimie et biolog ic moleculaire/Editions scientifiques et medicales Elsevier SAS.