10. Connecting bacterial replication to the cell cycle

12.10 Connecting bacterial replication to the cell cycle

Key terms defined in this section
Multiforked chromosome (in bacterium) has more than one replication fork, because a second initiation has occurred before the first cycle of replication has been completed.

Bacteria have two links between replication and cell growth:



  • The frequency of initiation of cycles of replication is adjusted to fit the rate at which the cell is growing.
  • The completion of a replication cycle is connected with division of the cell.

The rate of bacterial growth is assessed by the doubling time, the period required for the number of cells to double. The shorter the doubling time, the faster the growth rate. E. coli cells can grow at rates ranging from doubling times as fast as 18 minutes to slower than 180 minutes. Because the bacterial chromosome is a single replicon, the frequency of replication cycles is controlled by the number of initiation events at the single origin. The replication cycle can be defined in terms of two constants:



  • C is the fixed time of ~40 minutes required to replicate the entire bacterial chromosome. Its duration corresponds to a rate of replication fork movement of ~50,000 bp/minute. (The rate of DNA synthesis is more or less invariant at a constant temperature; it proceeds at the same speed unless and until the supply of precursors becomes limiting.)
  • D is the fixed time of ~20 minutes that elapses between the completion of a round of replication and the cell division with which it is connected. This period may represent the time required to assemble the components needed for division.

(The constants C and D can be viewed as representing the maximum speed with which the bacterium is capable of completing these processes. They apply for all growth rates between doubling times of 18 and 60 minutes, but both constant phases become longer when the cell cycle occupies >60 minutes.)


A cycle of chromosome replication must be initiated a fixed time before a cell division, C + D = 60 minutes. For bacteria dividing more frequently than every 60 minutes, a cycle of replication must be initiated before the end of the preceding division cycle.




Figure 12.24 The fixed interval of 60 minutes between initiation of replication and cell division produces multiforked chromosomes in rapidly growing cells. Note that only the replication forks moving in one direction are shown; actually the chromosome is replicated symmetrically by two sets of forks moving in opposite directions on circular chromosomes.

Consider the example of cells dividing every 35 minutes. The cycle of replication connected with a division must have been initiated 25 minutes before the preceding division. This situation is illustrated in Figure 12.24, which shows the chromosomal complement of a bacterial cell at 5 Vminute intervals throughout the cycle.


At division (35/0 minutes), the cell receives a partially replicated chromosome. The replication fork continues to advance. At 10 minutes, when this "old" replication fork has not yet reached the terminus, initiation occurs at both origins on the partially replicated chromosome. The start of these "new" replication forks creates a multiforked chromosome.


At 15 minutes Vthat is, at 20 minutes before the next division Vthe old replication fork reaches the terminus. Its arrival allows the two daughter chromosomes to separate; each of them has already been partially replicated by the new replication forks (which now are the only replication forks). These forks continue to advance.


At the point of division, the two partially replicated chromosomes segregate. This recreates the point at which we started. The single replication fork becomes "old," it terminates at 15 minutes, and 20 minutes later there is a division. We see that the initiation event occurs 1 25/35 cell cycles before the division event with which it is associated.


The general principle of the link between initiation and the cell cycle is that, as cells grow more rapidly (the cycle is shorter), the initiation event occurs an increasing number of cycles before the related division. There are correspondingly more chromosomes in the individual bacterium. This relationship can be viewed as the cell’s response to its inability to reduce the periods of C and D to keep pace with the shorter cycle.


How does the cell know when to initiate the replication cycle? The initiation event occurs at a constant ratio of cell mass to the number of chromosome origins. Cells growing more rapidly are larger and possess a greater number of origins. The growth of the bacterium can be described in terms of the unit cell, an entity 1.7 µm long. A bacterium contains one origin per unit cell; a rapidly growing cell with two origins will be 1.7 V3.4 µm long. In terms of Figure 12.24, it is at the point 10 minutes after division that the cell mass has increased sufficiently to support an initiation event at both available origins (Donachie and Begg, 1970; Donachie et al., 1976).


How is cell mass titrated? An initiator protein could be synthesized continuously throughout the cell cycle; accumulation of a critical amount would trigger initiation. This explains why protein synthesis is needed for the initiation event. An alternative possibility is that an inhibitor protein might be synthesized at a fixed point, and diluted below an effective level by the increase in cell volume (for review see Donachie, 1993).




Reviews
Donachie, W. D. (1993). The cell cycle of E. coli. Ann. Rev. Immunol. 47, 199-230.

Research
Donachie, W. D. and Begg, K. J. (1970). Growth of the bacterial cell. Nature 227, 1220-1224.
Donachie, W. D., Begg, K. J., and Vicente, M. (1976). Cell length, cell growth and cell division. Nature 264, 328-333.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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