The N-terminal region of AfsR, a putative pleiotropic regulatory protein for antibiotic production in Streptomyces coelicolor A3(2), is homologous to RedD and Actil-ORF4, pathway-specific regulatory proteins required for the production of the antibiotics undecylprodigiosin (Red) and actinorhodin (Act), respectively. The recent identification of afsS, which lies immediately 3' of afsR and which stimulates antibiotic production when cloned at high copy number, questioned whether afsR was a pleiotropic regulatory gene. In this study we demonstrate that multiple copies of afsR can stimulate both Act and Red production and that, despite its homology, it cannot substitute for the pathway-specific regulatory genes. Moreover, an in-frame deletion that removed most of the afsR coding sequence resulted in loss of Act and Red production, and a marked reduction in the synthesis of the calcium-dependent antibiotic (CDA), but only under some (non-permissive) nutritional conditions. Although additional copies of afsR resulted in elevated levels of the actII-ORF4 and redD transcripts, transcription of the pathway-specific regulatory genes under non-permissive conditions was unaffected by deletion of afsR. While afsR may operate independently of the pathway-specific regulatory proteins to influence antibiotic production, the activity of ActII-ORF4 and of RedD under non-permissive conditions could depend on interaction with, or modification by, AfsR.
The tetralin biodegradation genes of Sphingomonas macrogolitabida strain TFA are clustered in two closely linked and divergent operons. To analyze expression of both operons under different growth conditions, transcriptional and translational gene fusions of the first genes of each operon to lacZ have been constructed in plasmids unable to replicate in Sphingomonas and integrated by recombination into the genome of strain TFA. Expression analysis indicated that the transcription of both genes is induced in similar ways by the presence of tetralin. Gene expression in both operons is also subjected to overimposed catabolic repression. Two additional genes named thnR and thnY have been identified downstream of thnCA3A4 genes. ThnR is similar to LysR-type regulators, and mutational analysis indicated that ThnR is strictly required for expression of the thn operons. Unlike other LysR-type regulators, ThnR does not repress its own synthesis. In fact, ThnR activates its own expression, since thnR is cotranscribed with the thnCA3A4 genes. ThnY is similar to the ferredoxin reductase components of dioxygenase systems and shows the fer2 domain, binding a Cys 4 [2Fe-2S] iron sulfur center, and the FAD-binding domain, common to those reductases. However, it lacks the NADbinding domain. Intriguingly, ThnY has a regulatory role, since it is also strictly required for expression of the thn operons. Given the similarity of ThnY to reductases and the possibility of its being present in the two redox states, it is tempting to speculate that ThnY is a regulatory component connecting expression of the thn operons to the physiological status of the cell.The organic solvent tetralin (1,2,3,4-tetrahydronaphthalene) is a bicyclic molecule composed of an aromatic and an alicyclic moiety, which share two carbon atoms. Tetralin is widely used as a degreasing agent and solvent for fats, resins, and waxes, as a substitute for turpentine in paints, lacquers, and shoe polishes, and also in the petrochemical industry in connection with coal liquefaction (19). A concentration of tetralin higher than 100 M inhibits bacterial growth (44). Its toxicity is partly due to its lipophilic character, which results in its accumulation in the cell membranes, thus leading to changes in their structure and function (46,47). In addition, tetralin also forms toxic hydroperoxides in the cell (17).A few bacterial strains which are able to aerobically grow on tetralin as the only carbon and energy source have been isolated (44). By the identification of accumulated intermediates, several reports suggest that some bacteria, such as Pseudomonas stutzeri AS39 (43), initially hydroxylate and further oxidize the alicyclic ring whereas others, such as Corynebacterium sp. strain C125 (45), initially dioxygenate the aromatic ring, thus indicating that aerobic metabolism of tetralin can be performed in different ways. Metabolism of tetralin has been best characterized in Sphingomonas macrogolitabida strain TFA. Biodegradation of tetralin by the strain TFA involves initial...
SummaryThe genes required for tetralin biodegradation by Sphingomonas macrogolitabida strain TFA are clustered in two divergent and closely linked operons. ThnR, a LysR-type regulator, activates transcription from each operon in response to tetralin. The regulatory thnR gene is co-transcribed with the catabolic genes thnC, thnA3 and thnA4, resulting in positive autoregulation. ThnR binds with different affinity to two primary binding sites, designated B and C, in the intervening region between the two operons and makes additional contact with secondary sites that extend towards the promoters. In addition, ThnR may interact with itself when bound to each site via the formation of a DNA loop, as evidenced by the distortion of the DNA between the primary binding sites and the elimination of the higher-order complexes following the introduction of a half-turn of the DNA helix between the primary binding sites. Transcription from each promoter is not fully independent since mutations in each binding site affected transcription from both promoters. Based on these results, we propose a model of transcription activation that involves the formation of a complex structure by interactions between ThnR molecules bound to distant binding sites and favours transcription from one promoter to the detriment of the other.
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