Circadian rhythms govern changes in physiology and behavior over the course of a day and are based on a molecular clock. Katada and Sassone-Corsi now show that circadian changes in histone methylation at promoters are regulated by the methyltransferase MLL1 and linked to the recruitment of circadian transcription factors. The cover features the Prague astronomical clock, image from istockphoto.com. pp 1414–1421

Blacklow and colleagues present the structure of a dimeric Notch transcription complex with DNA, providing insights into how dimer binding sites with different spacers are recognized. On the cover, each suspension tower represents a Notch transcription complex bound to a DNA site and the bridge itself is the DNA. Image from iStockphoto, © Jeremy Edwards. pp 1312–1317

pp 1260–1262pp 1263–1265 Corrigendum 12 October 2010 In the description of the cover image originally published, credit for the original image was attributed incorrectly. The correct attribution should read, “Cover art by Marian Miller and Jack Griffith.” This error has been corrected for the online PDF and HTML versions of the caption.

During a cell cycle, the nucleus changes size, as do the number of nuclear pore complexes. Maeshima, Imamoto and colleagues now examine interphase nuclear pore complex formation and argue that nuclear growth and pore complex formation are regulated distinctly. The cover illustration, by Erin Boyle, shows an abstract view reminiscent of pores. (pp 1065–1071)

In some organisms, miRNA targeting leads to generation of secondary siRNAs, but why this happens for some miRNAs and not others has been unclear. Carrington and colleagues now find that Arabidopsis miRNAs that trigger an amplified response tend to be 22 nt rather than 21 nt in length. (pp 997–1003)

Podovirus P-SSP7 infects Prochlorococcus marinus, an abundant marine photosynthetic microorganism. Chiu and colleagues now examine the host infection process, revealing structural features that suggest that upon binding to the host cell, the tail fibers induce a cascade of structural alterations that trigger genome release. The cover photograph by Nicole Konforti depicts a marine environment along the coast of Belize. (pp 830–836)

Understanding how signal transduction pathways interact and are integrated remains a major challenge. This issue contains an Essay, Commentaries and a series of Reviews on the theme of Signal Integration. The cover "Network" is original artwork by Colleen Buzzard (http://www.colleenbuzzard.com), using ink on Braille paper. (pp 641–672)

Clade C is the most prevalent subtype of HIV-1 worldwide. The crystal structure of a clade C gp120 core is now described by Bjorkman and colleagues, in complex with its receptor CD4 and a CD4- induced antibody fragment. The work reveals that the antibody makes contacts with both gp120 and CD4. The cover features a glass sculpture of a viral particle by Luke Jerram (http://www.lukejerram.com/projects/glass_microbiology).pp 608–613

New World hemorrhagic fever arenaviruses are rodent-borne agents that cause severe human disease. The structure of Machupo virus surface glycoprotein 1 bound to the human transferrin receptor 1 is now presented by Harrison and colleagues. The structure resembles a butterfly, shown in the cover photograph by Richard Mann. pp pp 438–444

Adhesion by classical cadherins involves strand swapping dimerization via the N terminal EC1 domain. Shapiro and colleagues now find that T-cadherin forms an alternative cross dimer ("X dimer") and that this is an intermediate configuration that promotes formation of strand swapped dimer in classical cadherins. These forms are represented on the cover by Erin Boyle. pp 339–347 and pp 348–357

CDK8, found in some Mediator complexes, is now shown to be a positive regulator of transcription elongation, as opposed to initiation, at immediate early serum response genes. The image conveys the connection of the different factors in an unexpected arrangement. Based on an idea from J. Espinosa and Lacey Coover, original artwork by Colleen Buzzard (http://www.colleenbuzzard.com). pp 194-201

MicroRNAs are regulators of gene expression that have been identified in many species. Kai and Pasquinelli now review and discuss recent work showing that microRNA turnover is regulated, forming part of our web focus on RNA silencing (http://www.nature.com/nsmb/focus/rnasilencing). The cover, "Degradation," is original artwork by Colleen Buzzard (http://www.colleenbuzzard.com). pp 5-10