Fig. 2: A titration-based model can generate robust replication initiation cycles at low growth rates, but not at higher growth rates in the regime of overlapping replication forks.
From: Robust replication initiation from coupled homeostatic mechanisms
a–d The volume V(t), the number of initiator proteins Np(t) and titration sites Ns(t), the total concentration of initiator proteins [p]T(t), and the concentration of initiator proteins in the cytoplasm [p](t) as a function of time (in units of the doubling time of the cell τd) for τd = 2 h (a), τd = 35 min (b, c) and τd = 25 min (d), respectively. a When the number of initiator proteins per origin np(t) exceeds the number of titration sites per origin ns (yellow dashed line), the free concentration [p](t) rapidly rises to reach the threshold concentration \({K}_{{{{{{{{\rm{D}}}}}}}}}^{{{{{{{{\rm{ori}}}}}}}}}\) (blue dashed line), initiating a new round of replication. Due to the homogeneous distribution of titration sites on the chromosome of E. coli and the constant DNA replication rate, the number of titration sites then increases linearly in time. At low growth rates, new titration sites are synthesized faster than new initiator proteins and the free concentration [p](t) rapidly drops after initiation. After a fixed cycling time τcc (blue arrows) the cell divides. The initiation volume per origin v* (green dashed line) at low growth rates is constant in time. b However, in the regime of overlapping replication forks where the doubling time is smaller than the time to replicate the entire chromosome, τd < TC, new proteins are synthesized faster than new titration sites are formed. After a short period τb = 10 min (shaded red area) during which initiation at oriC is blocked via the protein SeqA, replication is reinitiated prematurely, dramatically raising the variation in the initiation volume (see Fig. 5c, green line). c, d Adding SeqA, which transiently blocks DnaA synthesis for a time τb = 10 min after replication initiation (shaded red area), prevents reinitiation events at high (d) but not at intermediate growth rates (c). (See Supplementary Table 1 for all parameters).
