Figure 1. Mature nucleosome arrays are formed in several stages.

Histone proteins are first loaded onto DNA to create non-canonical nucleosomal structures referred to as ‘prenucleosomes’ (top and second panels). The core histones, H2A, H2B, H3 and H4, associate with each other in heterodimers (H2A-H2B and H3-H4; top panel) and are deposited onto DNA in a specific order by proteins called histone chaperones (top blue box) to form an irregularly spaced prenucleosome array (dashed ovals, second panel). Mature nucleosomes are then formed, and also repositioned to ensure their even spacing on the DNA, by ATP-dependent chromatin remodelling factors (motor proteins; lower blue box). Maturation and repositioning were formerly thought to be achieved by a single enzymatic function (long arrow at left, bottom panel). Now, using mutant variants of a motor protein called Chd1, Torigoe et al. suggest that chromatin remodelling factors use not one but two distinct ATP-dependent activities to convert randomly deposited prenucleosomes into a mature array of evenly spaced nucleosomes (solid ovals; short arrows at right, third and bottom panels).