Transcriptional analysis of the recA gene in Streptomyces rimosus: identification of the new type of promoter

Streptomyces coelicolor macrodomain hydrolase SCO6735 cleaves thymidine-linked ADP-ribosylation of DNA

ADP-ribosylation is an ancient, highly conserved, and reversible covalent modification critical for a variety of endogenous processes in both prokaryotes and eukaryotes. ADP-ribosylation targets proteins, nucleic acids, and small molecules (including antibiotics). ADP-ribosylation signalling involves enzymes that add ADP-ribose to the target molecule, the (ADP-ribosyl)transferases; and those that remove it, the (ADP-ribosyl)hydrolases. Recently, the toxin/antitoxin pair DarT/DarG composed of a DNA ADP-ribosylating toxin, DarT, and (ADP-ribosyl)hydrolase antitoxin, DarG, was described. DarT modifies thymidine in single-stranded DNA in a sequence-specific manner while DarG reverses this modification, thereby rescuing cells from DarT toxicity. We studied the DarG homologue SCO6735 which is highly conserved in all Streptomyces species and known to be associated with antibiotic production in the bacterium S. coelicolor. SCO6735 shares a high structural similarity with the bacterial DarG and human TARG1. Like DarG and TARG1, SCO6735 can also readily reverse thymidine-linked ADP-ribosylation catalysed by DarT in vitro and in cells. SCO6735 active site analysis including molecular dynamic simulations of its complex with ADP-ribosylated thymidine suggests a novel catalytic mechanism of DNA-(ADP-ribose) hydrolysis. Moreover, a comparison of SCO6735 structure with ALC1-like homologues revealed an evolutionarily conserved feature characteristic for this subclass of macrodomain hydrolases.

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

The Mycobacterial LexA/RecA-Independent DNA Damage Response Is Controlled by PafBC and the Pup-Proteasome System

Mycobacteria exhibit two DNA damage response pathways: the LexA/RecA-dependent SOS response and a LexA/RecA-independent pathway. Using a combination of transcriptomics and genome-wide binding site analysis, we demonstrate that PafBC (proteasome accessory factor B and C), encoded in the Pup-proteasome system (PPS) gene locus, is the transcriptional regulator of the predominant LexA/RecA-independent pathway. Comparison of the resulting PafBC regulon with the DNA damage response of Mycobacterium smegmatis reveals that the majority of induced DNA repair genes are upregulated by PafBC. We further demonstrate that RecA, a member of the PafBC regulon and principal regulator of the SOS response, is degraded by the PPS when DNA damage stress has been overcome. Our results suggest a model for the regulation of the mycobacterial DNA damage response that employs the concerted action of PafBC as master transcriptional activator and the PPS for removal of DNA repair proteins to maintain a temporally controlled stress response.

ADP-ribosylation: new facets of an ancient modification

Rapid response to environmental changes is achieved by uni- and multicellular organisms through a series of molecular events, often involving modification of macromolecules, including proteins, nucleic acids and lipids. Amongst these, ADP-ribosylation is of emerging interest because of its ability to modify different macromolecules in the cells, and its association with many key biological processes, such as DNA-damage repair, DNA replication, transcription, cell division, signal transduction, stress and infection responses, microbial pathogenicity and aging. In this review, we provide an update on novel pathways and mechanisms regulated by ADP-ribosylation in organisms coming from all kingdoms of life.

Disruption of Macrodomain Protein SCO6735 Increases Antibiotic Production in Streptomyces coelicolor

ADP-ribosylation is a post-translational modification that can alter the physical and chemical properties of target proteins and that controls many important cellular processes. Macrodomains are evolutionarily conserved structural domains that bind ADP-ribose derivatives and are found in proteins with diverse cellular functions. Some proteins from the macrodomain family can hydrolyze ADP-ribosylated substrates and therefore reverse this post-translational modification. Bacteria and Streptomyces, in particular, are known to utilize protein ADP-ribosylation, yet very little is known about their enzymes that synthesize and remove this modification. We have determined the crystal structure and characterized, both biochemically and functionally, the macrodomain protein SCO6735 from Streptomyces coelicolor This protein is a member of an uncharacterized subfamily of macrodomain proteins. Its crystal structure revealed a highly conserved macrodomain fold. We showed that SCO6735 possesses the ability to hydrolyze PARP-dependent protein ADP-ribosylation. Furthermore, we showed that expression of this protein is induced upon DNA damage and that deletion of this protein in S. coelicolor increases antibiotic production. Our results provide the first insights into the molecular basis of its action and impact on Streptomyces metabolism.

Structure-function relationships of two paralogous single-stranded DNA-binding proteins from Streptomyces coelicolor: implication of SsbB in chromosome segregation during sporulation

The linear chromosome of Streptomyces coelicolor contains two paralogous ssb genes, ssbA and ssbB. Following mutational analysis, we concluded that ssbA is essential, whereas ssbB plays a key role in chromosome segregation during sporulation. In the ssbB mutant, ∼30% of spores lacked DNA. The two ssb genes were expressed differently; in minimal medium, gene expression was prolonged for both genes and significantly upregulated for ssbB. The ssbA gene is transcribed as part of a polycistronic mRNA from two initiation sites, 163 bp and 75 bp upstream of the rpsF translational start codon. The ssbB gene is transcribed as a monocistronic mRNA, from an unusual promoter region, 73 bp upstream of the AUG codon. Distinctive DNA-binding affinities of single-stranded DNA-binding proteins monitored by tryptophan fluorescent quenching and electrophoretic mobility shift were observed. The crystal structure of SsbB at 1.7 Å resolution revealed a common OB-fold, lack of the clamp-like structure conserved in SsbA and previously unpublished S-S bridges between the A/B and C/D subunits. This is the first report of the determination of paralogous single-stranded DNA-binding protein structures from the same organism. Phylogenetic analysis revealed frequent duplication of ssb genes in Actinobacteria, whereas their strong retention suggests that they are involved in important cellular functions.

SIGffRid: a tool to search for sigma factor binding sites in bacterial genomes using comparative approach and biologically driven statistics

Background

Many programs have been developed to identify transcription factor binding sites. However, most of them are not able to infer two-word motifs with variable spacer lengths. This case is encountered for RNA polymerase Sigma (σ) Factor Binding Sites (SFBSs) usually composed of two boxes, called -35 and -10 in reference to the transcription initiation point. Our goal is to design an algorithm detecting SFBS by using combinational and statistical constraints deduced from biological observations.

Results

We describe a new approach to identify SFBSs by comparing two related bacterial genomes. The method, named SIGffRid (SIGma Factor binding sites Finder using R'MES to select Input Data), performs a simultaneous analysis of pairs of promoter regions of orthologous genes. SIGffRid uses a prior identification of over-represented patterns in whole genomes as selection criteria for potential -35 and -10 boxes. These patterns are then grouped using pairs of short seeds (of which one is possibly gapped), allowing a variable-length spacer between them. Next, the motifs are extended guided by statistical considerations, a feature that ensures a selection of motifs with statistically relevant properties. We applied our method to the pair of related bacterial genomes of Streptomyces coelicolor and Streptomyces avermitilis. Cross-check with the well-defined SFBSs of the SigR regulon in S. coelicolor is detailed, validating the algorithm. SFBSs for HrdB and BldN were also found; and the results suggested some new targets for these σ factors. In addition, consensus motifs for BldD and new SFBSs binding sites were defined, overlapping previously proposed consensuses. Relevant tests were carried out also on bacteria with moderate GC content (i.e. Escherichia coli/Salmonella typhimurium and Bacillus subtilis/Bacillus licheniformis pairs). Motifs of house-keeping σ factors were found as well as other SFBSs such as that of SigW in Bacillus strains.

Conclusion

We demonstrate that our approach combining statistical and biological criteria was successful to predict SFBSs. The method versatility autorizes the recognition of other kinds of two-box regulatory sites.

Genetics of Streptomyces rimosus, the oxytetracycline producer

From a genetic standpoint, Streptomyces rimosus is arguably the best-characterized industrial streptomycete as the producer of oxytetracycline and other tetracycline antibiotics. Although resistance to these antibiotics has reduced their clinical use in recent years, tetracyclines have an increasing role in the treatment of emerging infections and noninfective diseases. Procedures for in vivo and in vitro genetic manipulations in S. rimosus have been developed since the 1950s and applied to study the genetic instability of S. rimosus strains and for the molecular cloning and characterization of genes involved in oxytetracycline biosynthesis. Recent advances in the methodology of genome sequencing bring the realistic prospect of obtaining the genome sequence of S. rimosus in the near term.

recA gene expression in a streptomycete is mediated by the unusual C-terminus of RecA protein

Streptomyces RecA proteins are characterized by a conserved, positively charged extension of unknown function appended at their C-termini. To investigate the function of this element, we introduced the Streptomyces rimosus recA gene and its mutant form encoding the protein with a C-terminal deletion into S. rimosus. Both transcript and protein levels were dramatically increased in the strain expressing the truncated gene compared to the strain bearing the wild-type recA, indicating involvement of the characteristic C-terminal extension in regulating the recA expression in Streptomyces. Considering that RecA acts as a major regulator of DNA damage response in bacteria, this mode of regulation is expected to have broader implications and significance that outreaches our current understanding of RecA autoregulation.

Bioinformatic identification of novel regulatory DNA sequence motifs in Streptomyces coelicolor

BACKGROUND

Streptomyces coelicolor is a bacterium with a vast repertoire of metabolic functions and complex systems of cellular development. Its genome sequence is rich in genes that encode regulatory proteins to control these processes in response to its changing environment. We wished to apply a recently published bioinformatic method for identifying novel regulatory sequence signals to gain new insights into regulation in S. coelicolor.

RESULTS

The method involved production of position-specific weight matrices from alignments of over-represented words of DNA sequence. We generated 2497 weight matrices, each representing a candidate regulatory DNA sequence motif. We scanned the genome sequence of S. coelicolor against each of these matrices. A DNA sequence motif represented by one of the matrices was found preferentially in non-coding sequences immediately upstream of genes involved in polysaccharide degradation, including several that encode chitinases. This motif (TGGTCTAGACCA) was also found upstream of genes encoding components of the phosphoenolpyruvate phosphotransfer system (PTS). We hypothesise that this DNA sequence motif represents a regulatory element that is responsive to availability of carbon-sources. Other motifs of potential biological significance were found upstream of genes implicated in secondary metabolism (TTAGGTtAGgCTaACCTAA), sigma factors (TGACN19TGAC), DNA replication and repair (ttgtCAGTGN13TGGA), nucleotide conversions (CTACgcNCGTAG), and ArsR (TCAGN12TCAG). A motif found upstream of genes involved in chromosome replication (TGTCagtgcN7Tagg) was similar to a previously described motif found in UV-responsive promoters.

CONCLUSIONS

We successfully applied a recently published in silico method to identify conserved sequence motifs in S. coelicolor that may be biologically significant as regulatory elements. Our data are broadly consistent with and further extend data from previously published studies. We invite experimental testing of our hypotheses in vitro and in vivo.