Cell biology is all about being in the right place at the right time. Get it wrong and, as has been shown for a number of disease states, the consequences can be dramatic. But how is protein localization regulated with such precision? And how can gene expression be timed to perfection?

The review by Jensen, Wang and Shapiro on page 167 of this issue tackles both of these questions, and describes how the specific expression and localization of DNA and proteins are regulated in the bacterium Caulobacter crescentus. During the Caulobacter life cycle, a cellular differentiation programme culminates in asymmetric cell division, which leads to the formation of two daughter cells with different fates. The exquisite timing of these cell-cycle and differentiation events is regulated at various levels, from the synthesis and stability of the proteins involved to their localization and activity — an amazing feat given that Caulobacter is just one micron in length.

When considering biological space and time, DNA and proteins are not the only important factors. Subcellular localization is also crucial for messenger RNAs, the translation and stability of which may depend on it. Much of this information is provided by specific mRNA-binding proteins, and these are reviewed by Dreyfuss, Kim and Kataoka (page 195). On a larger scale, whole cells can also alter their locality — during the processes that shape embryonic development, for example. The Snail superfamily of zinc-finger transcription factors is involved in many of these processes and, as M. Angelo Nieto discusses on page 155, its members are regulated in varying ways. A host of signalling pathways induces the expression of Snail family members, and these proteins, in turn, are involved in processes ranging from the formation of appendages to the signalling cascade that confers left–right identity.