DNA repair

Conserved modes of recruitment of ATM, ATR and DNA-PKcs to sites of DNA damage. Falck, J. et al. Nature 2 Mar 2005 (10.1038/nature03442)

The phosphoinositide-3-kinase-related protein kinase (PIKK)- family members ATM, ATR and DNA-PKcs are activated in response to DNA damage and recruited to sites of DNA damage by individual partner proteins. Falck et al. have now identified a conserved interaction motif in the C terminus of each partner protein — Nbs1, ATRIP and Ku80, respectively — that is essential for the recruitment of these PIKKs as well as for the downstream signalling events.

Chromosome segregation

Dissociation of cohesin from chromosome arms and loss of arm cohesion during early mitosis depends on phosphorylation of SA2. Hauf, S. et al. PLoS Biol. 3, e69 (2005)

Shugoshin prevents dissociation of cohesin from centromeres during mitosis in vertebrate cells. McGuinness, B. E. et al. PLoS Biol. 3, e86 (2005)

During mitosis, sister chromatids are held together by cohesin, which is cleaved in anaphase, triggering chromatid segregation. However, most cohesin is removed from the chromosome arms, but not from the centromere, before anaphase. One study now shows that phosphorylation of the cohesin subunit SA2 is required for cohesin removal from chromosome arms in early mitosis. So what safeguards centromeric cohesin? Shugoshin, according to a second study, as this protein seems to protect centromeric SA2 from phosphorylation to prevent premature chromatid segregation.

Plant biology

RNA polymerase IV directs silencing of endogenous DNA. Herr, A. J. et al. Science 3 Feb 2005 (10.1126/science.1106910)

Plant nuclear RNA polymerase IV mediates siRNA and DNA methylation-dependent heterochromatin formation. Onodera, Y. et al. Cell 10 Feb 2005 (10.1016/S0092867405001510)

Plants encode catalytic subunits of a fourth RNA polymerase, Pol IV, the function of which has remained unknown until now. Data from two studies now indicate that Pol IV causes small interfering (si)RNA-mediated gene silencing and facultative heterochromatin formation. These events are associated with RNA-directed DNA methylation, which requires siRNAs for targeting specific de novo DNA-methylation events. The production of siRNAs might be mediated by a mechanism that involves Pol IV, although its precise role is yet to be determined.