A Complex Genetic Switch Involving Overlapping Divergent Promoters and DNA Looping Regulates Expression of Conjugation Genes of a Gram-positive Plasmid
Figure 9
Model of the different layers contributing to the genetic switch controlling expression of the pLS20 conjugation genes.
A. RNA polymerase acts itself as a switch because it is unable to bind simultaneously to both of the two overlapping and divergently oriented promoters. Consequently, RNA polymerase (the brown ellipse shaped form) binds only one promoter at a time resulting in transcription of only the gene(s) controlled by this promoter. B. RcoLS20 generates a self-sustaining positive feedback loop by activating transcription from its own promoter (Pr) (left panel). This, combined with the simultaneous repression of the divergent conjugation promoter (Pc), results in conjugation being maintained effectively in the “OFF” state. Relief of RcoLS20-mediated repression of the Pc promoter results in activation of the conjugation genes (right panel). In addition, this interrupts the auto-stimulation of the Pr promoter, preventing further synthesis of RcoLS20, which in turn will contribute in pushing and maintaining conjugation in the “ON” state. The negative auto-regulatory loop of RcoLS20 that probably functions to keep RcoLS20 within a low concentration range (see text) is not presented. C. DNA looping results in a high local concentration of RcoLS20, increasing specificity and affinity that dampens transcriptional fluctuations between and within individual cells (left panel). This would contribute to tight repression of the Pc promoter, keeping conjugation in the “OFF” state under conditions antithetic to conjugation without compromising the ability to switching rapidly to a high expression state (i.e. “ON”, right panel) of the conjugation genes when appropriate conditions occur. rcoLS20 and gene 28, -the first gene of the conjugation operon-, are indicated with large red and blue arrows, respectively. The same coloring scheme is used for the corresponding promoters (rectangular) and transcripts (thin broken arrows). Activation and repression of transcription are indicated with continuous black lines ending in an arrow and a “T” shape, respectively. The red cylindrical structures, which may reflect one or two RcoLS20 tetramers, represent the RcoLS20 oligomer mediating DNA looping.