All of life is a balancing act. Whether you’re walking the tight-rope in a circus, or the epigenome regulating gene expression its all about treading a fine line. Recently a team from the Institute of Medical Biology in Singapore found, while examining frog development, that there is an optimal ratio of the repressive histone H3 and it’s activating linker buddy histone H1, which is critical at certain time points to tip the scales for proper development and differentiation. Here’s what went down:
- Frog (Xenopus) embryos that were depleted of histones H3 and H3.3 failed to produce the gene expression profiles for an important developmental step (gastrulation of the mesoderm).
- However, the team found that this transcriptional activation did not correlate with the placement of the repressive H3.3 in promoters.
- The team was able to partially rescue the phenotype, but interestingly this was done by concurrently depleting the activating histone H1 (the linker).
Figure 6. Lim et. al., Development 140, 2014
The Singapore group discovered a tipping point in development that depends on the ratio, rather than absolute amounts, of histone placement. The authors say this makes perfect sense given the spatial and temporal complexity behind the gene expression patterns required for proper development and differentiation. And, in terms of take home message, it looks like the histone code isn’t just written by modifications and variants, but also the respective ratio of their placement in the genome.
Learn all about tipping the scales of the epigenome in Development, March 2014