The initiator titration model: computer simulation of chromosome and minichromosome control.

The initiator titration model was formulated to explain the initiation control of the bacterial chromosome. In particular, features concerning the replication behaviour of minichromosomes, such as their high copy number and Escherichia coli's ability to coinitiate chromosome and many minichromosome origins, were considered during the formulation of the model. The model is based on the initiator protein DnaA and its binding sites, DnaA boxes, in oriC, in the dnaA promoter and at other positions on the chromosome. Another important factor in the model is the eclipse period created by the hemimethylation of a new oriC which makes it refractory to initiation. The model was analysed by computer simulations using a stochastic approach varying the different input parameters, and the resulting computer cells were compared with data on living E. coli cells. Here we present the outcome of a few of these simulations concerning the eclipse period, in silico-shift experiments blocking initiation or elongation of replication, and introduction of minichromosomes into the computer cells. We also discuss the synthesis of DnaA protein in the computer cells. From our simulations, we conclude that, whether true or not, the model can mimic the in vivo initiation control of E. coli.