Nitrogen regulatory protein C (NtrC) contacts a bacterial RNA polymerase from distant enhancers by means of DNA loops and activates transcription by allowing polymerase to gain access to the template DNA strand. It was shown that NtrC from Salmonella typhimurium must build large oligomers to activate transcription. In contrast to eukaryotic enhancer-binding proteins, most of which must bind directly to DNA, some NtrC dimers were bound solely by protein-protein interactions. NtrC oligomers were visualized with scanning force microscopy. Evidence of their functional importance was provided by showing that some inactive non-DNA-binding and DNA-binding mutant forms of NtrC can cooperate to activate transcription.

No abstract

No abstract

Many bacteria respond to a lack of iron in the environment by synthesizing siderophores, which act as iron-scavenging compounds. Fluorescent pseudomonads synthesize strain-specific but chemically related siderophores called pyoverdines or pseudobactins. We have investigated the mechanisms by which iron controls expression of genes involved in pyoverdine metabolism in Pseudomonas aeruginosa. Transcription of these genes is repressed by the presence of iron in the growth medium. Three promoters from these genes were cloned and the activities of the promoters were dependent on the amounts of iron in the growth media. Two of the promoters were sequenced and the transcriptional start site were identified by S1 nuclease analysis. Sequences similar to the consensus binding site for the Fur repressor protein, which controls expression of iron-repressible genes in several gram-negative species, were not present in the promoters, suggesting that they are unlikely to have a high affinity for Fur. However, comparison of the promoter sequences with those of iron-regulated genes from other Pseudomonas species and also the iron-regulated exotoxin gene of P. aeruginosa allowed identification of a shared sequence element, with the consensus sequence (G/C)CTAAAT-CCC, which is likely to act as a binding site for a transcriptional activator protein. Mutations in this sequence greatly reduced the activities of the promoters characterized here as well as those of other iron-regulated promoters. The requirement for this motif in the promoters of iron-regulated genes of different Pseudomonas species indicates that similar mechanisms are likely to be involved in controlling expression of a range of iron-regulated genes in pseudomonads.

No abstract