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The mechanism underlying the LuxR repression of the type 3 secretion system (T3SS) system in Vibrio harveyi is determined. The studies have defined the genes in the Vibrio harveyi quorum sensing (QS) regulon and have analyzed a mechanism showing how multiple regulators are linked in series to direct the expression of QS target genes specifically at low or high cell density.

Gonococci secrete chromosomal DNA into the extracellular environment using a type IV secretion system ( T4SS). The secreted DNA acts in natural transformation and initiates biofilm development. Although the DNA and its effects are detectable, structural components of the T4SS are present at very low levels, suggestive of uncharacterized regulatory control. We sought to better characterize the expression and regulation of T4SS genes and found that the four operons containing T4SS genes are transcribed at very different levels. Increasing transcription of two of the operons through targeted promoter mutagenesis did not increase DNA secretion. The stability and steady-state levels of two T4SS structural proteins were affected by a homolog of tail-specific protease. An RNA switch was also identified that regulates translation of a third T4SS operon. The switch mechanism relies on two putative stem-loop structures contained within the 5′ untranslated region of the transcript, one of which occludes the ribosome binding site and start codon. Mutational analysis of these stem loops supports a model in which induction of an alternative structure relieves repression. Taken together, these results identify multiple layers of regulation, including transcriptional, translational and post-translational mechanisms controlling T4SS gene expression and DNA secretion. [ABSTRACT FROM AUTHOR]