CellWallAssembly During Inhibition ofDNA Synthesis in Streptococcus faecium

culture doubling inmass every 64min,DNA synthesis was inhibited, andeventually cell division stopped. Thegrowthsites formed before andafter inhibition ofDNA synthesis enlarged until theycontained about0.25,um3ofcell volume, atwhichpoint theyceased toincrease insize. Whenthesesites approached this0.25-P,m3 limit, new sites were initiated; this result hadalsobeenobserved inuntreated cells undergoing a large range of exponential-phase mass doubling times. Thus, regardless ofwhether chromosome replication isinhibited or uninhibited, sites havethesame finite capacity to enlarge toabout0.25P,m3, andwhenthiscapacity isreached, new sites are initiated. Although initiation ofnew growthsites seems tobeindependent of normal chromosome replication, these results confirm previous studies showing thatchromosome replication isnecessaryfortheterminal eventsofgrowthsite development whichresult inthedivision ofa site into twoseparatepoles. Two classes ofmodels fortheregulation ofgrowth site initiation arediscussed. Thesurface ofthegram-positive coccus Streptococcus faecium ATCC 9790growsat discrete growth sites inwhichsepta separate and expand into pairs ofpolar caps. Theinitial event intheformation ofanewgrowthsite isthe synthesis ofanannular ridge ofmaterial onthe internal surface ofthecell wall. Thisridge forms a nascent septumwhichisusually assembled directly under araised bandfoundontheexternalsurface ofthecell wall(wall band; Fig. 1A). Thisisfollowed bythecreation ofadivision furrow whichcleaves thewall bandandunderlyingseptumandallows theseptumtoseparate into twonewlayers ofperipheral wall. Thetwo bandsformedbythis initial cleavage serve to delineate thewallsynthesis thatoccurs ina single site (Fig. 1AandB). Recently, wehaveusedthese wallbandsas markers tostudy thenumberandgeometry of growth sites seeninplatinum-carbon replicas of balanced exponential-phase