  In the simplest view of transcriptional regulation, the expression of a gene
is turned on or off by changes in the concentration of a transcription factor
(TF). We use recent data on noise levels in gene expression to show that it
should be possible to transmit much more than just one regulatory bit.
Realizing this optimal information capacity would require that the dynamic
range of TF concentrations used by the cell, the input/output relation of the
regulatory module, and the noise levels of binding and transcription satisfy
certain matching relations. This parameter-free prediction is in good agreement
with recent experiments on the Bicoid/Hunchback system in the early Drosophila
embryo, and this system achieves ~90% of its theoretical maximum information
transmission.
