Among transcription factors, several groups have been identified according to their conserved motifs and their modes of DNA binding such as helix-turn-helix, zinc-fingers, leucinezipper, homeodomain, and ␤-sheet DNA-binding proteins (2, 3). The most studied and best characterized is the HTH 1 group (1, 4 -8) in which the conserved DNA recognition motif consists of an ␣-helix, a turn, and a second ␣-helix, often called the "recognition" helix as it is the part of the HTH motif that fits into the DNA major groove. Generally, HTH proteins bind as dimers, 2-fold symmetric DNA sequences in which each monomer recognizes a half-site. This group is now considered as a reference for understanding the general rules that govern protein-DNA interactions (9, 10) and has also become a favorite target for evolutionary studies (8,11).Among HTH transcriptional regulators, families have been identified throughout sequence comparisons and phylogenetic, structural, and functional analyses focused on DNA-binding domains and almost exclusively on the HTH structure, which is the only active motif that shows strong similarities among all members of the group (1, 4, 6 -8, 11). These comparative studies have led to the determination of a specific HTH consensus pattern or signature for each family, providing the basis for a simple method of classification and detection of new members (12).The lack of significant similarity among regions involved in effector binding or oligomerization systematically excludes these domains during families signature establishment, although they have important roles in the regulating process. In fact, it is often the oligomerization between regulatory subunits and/or the conformational changes due to the binding or the removal of the inducing/repressing molecule that allows correct HTH motif disposition and the subsequent DNA binding ability of the whole regulatory protein. The link between the two regions is therefore more intimate than it first appears from a unique amino acids comparison and may also be reflected in the DNA operator sequences, the third structural element involved in gene regulation.To argue for the existence of a link between regions involved in the regulating process, we analyzed the HTH GntR family of bacterial regulators. As determined thus far, the family comprises about 270 members distributed among the most diverse bacterial groups and regulating the most various biological processes. This family was first described by Haydon and Guest in 1991 (1) and was named after GntR, the repressor of the gluconate operon in Bacillus subtilis (13,14). Our interest in the properties of these bacterial regulators arises from the identification by our laboratory of the xlnR gene (15) in which chromosomal disruption in Streptomyces lividans relieves various extracellular enzymatic systems from glucose repression.The first purpose of this report is to present, 10 years after the first comparative study, an update of the GntR family description. Moreover, we decided to analyze the full-length sequenc...

Members of the soil-dwelling prokaryotic genus Streptomyces produce many secondary metabolites, including antibiotics and anti-tumour agents. Their formation is coupled with the onset of development, which is triggered by the nutrient status of the habitat. We propose the first complete signalling cascade from nutrient sensing to development and antibiotic biosynthesis. We show that a high concentration of N-acetylglucosamine-perhaps mimicking the accumulation of N-acetylglucosamine after autolytic degradation of the vegetative mycelium-is a major checkpoint for the onset of secondary metabolism. The response is transmitted to antibiotic pathway-specific activators through the pleiotropic transcriptional repressor DasR, the regulon of which also includes all N-acetylglucosamine-related catabolic genes. The results allowed us to devise a new strategy for activating pathways for secondary metabolite biosynthesis. Such 'cryptic' pathways are abundant in actinomycete genomes, thereby offering new prospects in the fight against multiple drug-resistant pathogens and cancers.

SummaryMembers of the soil-dwelling, sporulating prokaryotic genus Streptomyces are indispensable for the recycling of the most abundant polysaccharides on earth (cellulose and chitin), and produce a wide range of antibiotics and industrial enzymes. How do these organisms sense the nutritional state of the environment, and what controls the signal for the switch to antibiotic production and morphological development? Here we show that high extracellular concentrations of N-acetylglucosamine, the monomer of chitin, prevent Streptomyces coelicolor progressing beyond the vegetative state, and that this effect is absent in a mutant defective of N-acetylglucosamine transport. We provide evidence that the signal is transmitted through the GntR-family regulator DasR, which controls the N-acetylglucosamine regulon, including the pts genes ptsH, ptsI and crr needed for uptake of N-acetylglucosamine. Deletion of dasR or the pts genes resulted in a bald phenotype. Binding of DasR to its target genes is abolished by glucosamine 6-phosphate, a central molecule in N-acetylglucosamine metabolism. Extracellular complementation experiments with many bld mutants showed that the dasR mutant is arrested at an early stage of the developmental programme, and does not fit in the previously described bld signalling cascade. Thus, for the first time we are able to directly link carbon (and nitrogen) metabolism to development, highlighting a novel type of metabolic regulator, which senses the nutritional state of the habitat, maintaining vegetative growth until changing circumstances trigger the switch to sporulation. Our work, and the model it suggests, provide new leads towards understanding how microorganisms time developmental commitment.

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