Rob is regarded as a constitutively expressed protein, although little is known about how rob gene is regulated. We show here by reverse transcription-PCR that the transcriptional levels of rob are strongly down-regulated in response to the superoxide-generating agent paraquat (PQ). Repression reached a maximum of 20-fold after 10 min exposure at 10 microM PQ. The magnitude of rob repression was comparable to that of induction quantified for the most sensitive SoxS targets. beta-Galactosidase expression with the rob2::lacZ transcriptional fusion indicates that down-regulation of rob expression takes place, at least in part, at the level of transcription initiation. Moreover, ca. 50% of the rob mRNA was degraded in <1 min after the addition of rifampin to inhibit transcription. This intrinsic short half-life, which is of obvious benefit for a rapid down-regulation after transcription ceases, was unaffected by the addition of PQ. No repression was observed in a soxR-null strain, indicating that the rob transcript level might be negatively modulated by the intracellular amounts of SoxS protein. Gel retardation assays support the idea that in vivo SoxS would block rob transcription directly.