Discontinuity in DNA replication during expression of accumulated initiation potential in dnaA mutants of Escherichia coli

Distinguishing between linear and exponential cell growth during the division cycle: single-cell studies, cell-culture studies, and the object of cell-cycle research

BACKGROUND

Two approaches to understanding growth during the cell cycle are single-cell studies, where growth during the cell cycle of a single cell is measured, and cell-culture studies, where growth during the cell cycle of a large number of cells as an aggregate is analyzed. Mitchison has proposed that single-cell studies, because they show variations in cell growth patterns, are more suitable for understanding cell growth during the cell cycle, and should be preferred over culture studies. Specifically, Mitchison argues that one can glean the cellular growth pattern by microscopically observing single cells during the division cycle. In contrast to Mitchison's viewpoint, it is argued here that the biological laws underlying cell growth are not to be found in single-cell studies. The cellular growth law can and should be understood by studying cells as an aggregate.

RESULTS

The purpose or objective of cell cycle analysis is presented and discussed. These ideas are applied to the controversy between proponents of linear growth as a possible growth pattern during the cell cycle and the proponents of exponential growth during the cell cycle. Differential (pulse) and integral (single cell) experiments are compared with regard to cell cycle analysis and it is concluded that pulse-labeling approaches are preferred over microscopic examination of cell growth for distinguishing between linear and exponential growth patterns. Even more to the point, aggregate experiments are to be preferred to single-cell studies.

CONCLUSION

The logical consistency of exponential growth--integrating and accounting for biochemistry, cell biology, and rigorous experimental analysis--leads to the conclusion that proposals of linear growth are the result of experimental perturbations and measurement limitations. It is proposed that the universal pattern of cell growth during the cell cycle is exponential.

Overreplication of the origin region in the dnaB37 mutant of Bacillus subtilis: postinitiation control of chromosomal replication

When the Bacillus subtilis dnaB37 mutant, defective in initiation, is returned to permissive temperature after accumulation of initiation proteins at 45 degrees C, we have shown, by extensive DNA.DNA hybridization analysis, that the origin region is replicated in excess (approximately 2-fold). However, this replication is limited to a region of about 120-175 kilobases on either side of the origin. This has been confirmed by autoradiographic analysis of the overreplicated region. During the second round of synchronized replication at 30 degrees C, replication in fact appears to resume from the stalled forks on either side of the origin. We propose that in B. subtilis, in addition to a first level of control at the origin, a second level of control exists downstream of the origin in order to limit overreplication of the chromosome. These two controls might normally be tightly coupled. We suggest that the second level of control is exerted through the reversible inhibition of replisome movement at specific regions on either side of the origin.

Overinitiation of replication of the Escherichia coli chromosome from an integrated runaway-replication derivative of plasmid R1

A 16-base-pair fragment, deletion of which completely inactivated oriC, was replaced by a temperature-dependent runaway-replication derivative (the copy number of which increases with temperature) of the IncFII plasmid R1. The constructed strains were temperature sensitive, and flow cytometry revealed a severalfold increase in the DNA/mass ratio following shifts to nonpermissive temperatures. The cell size distribution was broader in the constructed strains relative to that in the wild type because of asynchrony between the chromosome replication and cell division cycles. This difference was more pronounced for counterclockwise initiation of chromosomal replication, in which small DNA-less cells and long filaments were abundant. Following a temperature shift the cell size distributions became even more broad, showing that changes in the frequency of chromosomal replication affect cell division and emphasizing the interplay between these two processes.