REGULATORY NOISE IN PROKARYOTIC PROMOTERS - HOW BACTERIA LEARN TO RESPOND TO NOVEL ENVIRONMENTAL SIGNALS

Various features of the regulation of pathways for biodegradation of r ecalcitrant compounds by Pseudomonas provide insights into the mechani sms by which operons evolve to acquire conditionally active promoters that permit the corresponding genes to be transcribed only when requir ed. The 'regulatory noise hypothesis' proposes that transcriptional co ntrol systems develop responsiveness to new signals due to the leakine ss and lack of specificity of preexisting promoters and regulators, Wh en needed, these may become more specific through suppression of undes irable signals and further fine-tuning of the recruited proteins to in teract with distinct chemicals, This hypothesis is supported by the so phisticated regulation of sigma(54)-dependent promoters of the TOL (to luene biodegradation) operons, which can be activated to various degre es by heterologous proteins. Such 'illegitimate' activation is suppres sed by bent DNA structures, either static or protein induced, between promoter core elements, Therefore, not only the regulators but also th e DNA sequences participate in the process that gives rise to novel sp ecificities.