Coordinating DNA replication with cell division: Lessons from outgrowing spores

Progress in solving the long-standing puzzle of how a cell coordinates chro mosome replication with cell division is significantly aided by the use of synchronous cell populations. Currently three systems are employed for obta ining such populations the Escherichia coli 'baby machine', the development ally-controlled cell cycle of Caulobacter crescentus, and Bacillus subtilis germinated and outgrowing spores. This review examines our current underst anding of the relationship between replication and division and how the use of B. subtilis outgrowing spores and, more recently its combination with i mmunofluorescence microscopy, has contributed significantly to this importa nt area of biology. About 20 years ago, and also more recently, this system was used to show convincingly that termination of DNA replication is not e ssential for a central septum to form, raising the possibility that the ear ly stages of division occur well before termination. It has also been demon strated that there is no major synthesis of the division initiation protein s, FtsZ and DivIB. linked to initiation, progression or completion of the f irst round of chromosome replication accompanying spore outgrowth. This has led to the suggestion that the primary link between chromosome replication and cell division at midcell is not likely to occur through a control over the levels of these proteins. Very recent work has employed a combination of the use of B. subtilis outgrowing spores with immunofluorescence microsc opy to investigate the relationship between midcell Z ring assembly and the round of chromosome replication linked to it. The results of this work sug gest a role for initiation and progression into the round of replication in blocking midcell Z ring formation until the round is complete or almost co mplete, thereby ensuring that cell division occurs between two equally-part itioned chromosomes. (C) 2001 Societe francaise de biochimie et biologie mo leculaire/ Editions scientifiques et medicales Elsevier SAS.