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#arabinose

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Gene regulation in Prokaryotes

Full activation of ara operon requires

The araBAD operon exemplifies dual positive control integrating substrate availability and catabolite status. In absence of arabinose, AraC represses by looping DNA between araO2 and araI1, occluding RNA polymerase. Arabinose binding converts AraC into an activator that occupies araI1 and araI2, contacting polymerase and inducing bending. Maximal transcription additionally needs glucose starvation signaling. Elevated cAMP binds CAP, which recognizes site upstream of araI, enhancing polymerase alpha-CTD interaction. Both arabinose and CAP-cAMP together relieve looping and stimulate initiation, enabling catabolism only when preferred sugars are depleted.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7: Prokaryotic Transcription Regulation, ara Operon

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Gene regulation in Prokaryotes

In presence of arabinose, AraC binds to

Presence of L-arabinose triggers major structural rearrangement within AraC regulatory protein. Sugar molecule binds deep pocket within N-terminal domain forming hydrogen bonds that retract N-terminal arm from dimerization interface, altering interdomain orientation and allosteric communication. Dimer consequently loses high affinity for distal O2 site, abandons loop, and preferentially occupies adjacent half-sites araI1 centered minus 100 and araI2 at minus 50 directly upstream of PBAD promoter. Occupancy of I1-I2 positions AraC activation surface near minus45 facilitating direct interaction with sigma70 subunit and alpha C-terminal domain of RNA polymerase, breaking repressive loop and potently stimulating transcription initiation of araBAD catabolic genes.

Ref: Wikipedia L-arabinose operon – arabinose-bound AraC binds I1 and I2 acting as activator for araBAD expression.

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Gene regulation in Prokaryotes

In absence of arabinose, AraC binds to

Without L-arabinose, AraC apoprotein adopts conformation strongly promoting looping-mediated repression of araBAD operon. Dimer bridges high-affinity araO2 site centered approximately minus 280 upstream and araI1 site at minus 106, with intervening 210 base pairs bent into looped architecture demonstrated by electron microscopy. Crystallographic studies show N-terminal arms dimerize antiparallel, stabilizing loop topology. Looped conformation buries PBAD minus35 promoter element and occludes CAP-cAMP binding site, preventing RNA polymerase holoenzyme initiation. Approximately twenty AraC molecules per bacterial cell suffice to maintain tight repression, ensuring low basal expression until arabinose reaches threshold concentration sufficient to disrupt loop architecture.

Ref: Science DNA Looping by AraC – apo-AraC prefers looping between araO2 and araI1 repressing araBAD.

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