Development

Cell lineage transport: a mechanism for molecular gradient formation. Ibañes, M. et al. Mol. Syst. Biol. 17 October 2006 (doi: 10.1038/msb4100098)

These authors provide a new model for the formation of morphogenetic gradients. Using a mathematical simulation of cell division and growth dynamics, they show how mRNAs and non-secreted proteins can form robust gradients when dividing cells dilute and transport their contents. The biological relevance of this model, which the authors call 'cell lineage transport', was demonstrated by experimental evidence that Hoxd13 mRNA forms a gradient during vertebrate limb development as a result of such a mechanism.

RNA world

A two-hit trigger for siRNA biogenesis in plants. Axtell, M. J. et al. Cell 127, 565–577 (2006)

In some of their targets, plant microRNAs (miRNAs) direct cleavage to define one end of RNAs that are further processed to produce phased small interfering RNAs (siRNAs). In a search for other features that are required for this type of siRNA production, Axtell and colleagues showed that the presence of two cleavage sites for moss miR390 results in such an outcome. In an Arabidopsis thaliana siRNA locus, two miR390 sites are also required for phased siRNA production. However, cleavage is not needed at one of these sites, demonstrating an alternative role for some miRNA target sites.

Gene expression

Tandem riboswitch architectures exhibit complex gene control functions. Sudarsan, N. et al. Science 314, 300–304 (2006)

Bacterial gene expression is often controlled by riboswitches — structured RNAs in the UTRs of an mRNA that repress gene expression when bound to the relevant metabolite. These authors have discovered a new level of complexity in such systems — the metE mRNA of Bacillus clausii contains two riboswitches that bind two different metabolites, either of which can turn off expression. This Boolean NOR logic is similar to the mechanism that controls metE expression in Escherichia coli, but, in that species, proteins are used to sense the presence of both metabolites.

Epigenetics

Recent assembly of an imprinted domain from non-imprinted components. Rapkins, R. W. et al. PLoS Genet. 2, e182 (2006)

Mammals are the only animals with imprinted genes, but how and when genomic imprinting arose is unclear. The authors compared the arrangement of genes in placental mammals at the imprinted Prader–Willi/Angelman domain with that in other groups of mammals. In monotremes and marsupials, the components of the domain — genes and regulatory regions — are distributed on different chromosomes, as they are in non-mammals. Expression studies confirmed that this locus is not imprinted in marsupials and monotremes, showing that an imprinted domain was assembled relatively recently from non-imprinted components.