DNA is made (green diagonals) during the reductive phase before respiration begins.

Klevecz/NAS

New results from Robert Klevecz, Douglas Murray, and colleagues (City of Hope Medical Center, Duarte, CA) reveal that DNA is synthesized when it is least likely to be hit with oxidative damage.

Damaging oxidizing agents are produced by respiration. In yeast, respiration is periodic—it alternates in ∼40-min cycles with a stage of nonrespiration called the reductive phase. This oscillation was detected decades ago, but it was never thought to be connected to cell cycle progression. Now, Klevecz shows that both S-phase entry and transcription are controlled by the respiratory oscillation.

Microarray analysis revealed that nearly 90% of transcribed yeast genes were maximally transcribed in two peaks during the reductive phase—one early and one late. The transcription of fewer than 700 genes, in contrast, peaked during respiration.

The respiration cycle also gated entry into S-phase. Although only 10% of cells in any given respiration cycle entered S-phase, all those that did entered just as the reductive phase began. Klevecz hypothesizes that the timing strategy evolved when earth's atmosphere changed from a reducing to an oxidizing environment. “Single-stranded DNA is very susceptible to oxidative damage,” he says. “So the idea would be to avoid damage to DNA, and perhaps to RNA as well. If it's not broken, you don't have to fix it.” He is still searching for a synchronized mammalian cell culture system to test whether oscillations are widely conserved.

Klevecz warns that the design of typical treated-versus-control experiments must involve time series sampling and take phase into account, or else significant differences might be due solely to differences in phase. “The cell is in essence an oscillator,” he says. “Normal rules for cause and effect break down in systems that are oscillatory.” ▪

Reference:

Klevecz, R.R., et al. 2004. Proc. Natl. Acad. Sci. USA. 10.1073/pnas.0306490101.