  DNA looping has been observed to enhance and suppress transcriptional noise
but it is uncertain which of these two opposite effects is to be expected for
given conditions. Here, we derive analytical expressions for the main
quantifiers of transcriptional noise in terms of the molecular parameters and
elucidate the role of DNA looping. Our results rationalize paradoxical
experimental observations and provide the first quantitative explanation of
landmark individual-cell measurements at the single molecule level on the
classical lac operon genetic system [Choi et al., Science 322, 442-446 (2008)].
