Silencing of levB, the second structural gene of the tricistronic levansucrase operon encoding the endolevanase LevB, decreases the level of levansucrase expression. Conversely, independent expression of levB greatly stimulates operon expression in Bacillus subtilis. This autogenous effect is mediated by the levB transcript, which carries an internal sequence (5'-AAAGCAGGCAA-3') involved in the enhancing effect. In vitro, the levB transcript displays an affinity to the N-terminal fragment of SacY (KD 0.2 µM), the regulatory protein that prevents transcription termination of levansucrase operon. This positive feed back loop leads to an increase in the operon expression when B. subtilis is growing in the presence of high sucrose concentrations. Under these conditions, extracellular levan synthesized by the fructosyl polymerase activity of levansucrase can be degraded mainly into levanbiose by the action of LevB. Levanbiose is neither taken up nor metabolized by the bacteria. This work modifies the present view of the status of levansucrase in B. subtilis physiology
