Two distantly homologous DnaG primases from Thermoanaerobacter tengcongensis exhibit distinct initiation specificities and priming activities

The application of thermophilic DNA primase TtDnaG2 to DNA amplification

For DNA replication in vivo, DNA primase uses a complementary single-stranded DNA template to synthesize RNA primers ranging from 4 to 20 nucleotides in length, which are then elongated by DNA polymerase. Here, we report that, in the presence of double-stranded DNA, the thermophilic DNA primase TtDnaG2 synthesizes RNA primers of around 100 nucleotides with low initiation specificity at 70 °C. Analysing the structure of TtDnaG2, we identified that it adopts a compact conformation. The conserved sites in its zinc binding domain are sequestered away from its RNA polymerase domain, which might give rise to the low initiation specificity and synthesis of long RNA segments by TtDnaG2. Based on these unique features of TtDnaG2, a DNA amplification method has been developed. We utilized TtDnaG2 to synthesize RNA primers at 70 °C after 95 °C denaturation, followed by isothermal amplification with the DNA polymerase Bst3.0 or phi29. Using this method, we successfully amplified genomic DNA of a virus with 100% coverage and low copy number variation. Our data also demonstrate that this method can efficiently amplify circular DNA from a mixture of circular DNA and linear DNA, thus providing a tool to amplify low-copy-number circular DNA such as plasmids.

Conserved TRAM Domain Functions as an Archaeal Cold Shock Protein via RNA Chaperone Activity

Cold shock proteins (Csps) enable organisms to acclimate to and survive in cold environments and the bacterial CspA family exerts the cold protection via its RNA chaperone activity. However, most Archaea do not contain orthologs to the bacterial csp. TRAM, a conserved domain among RNA modification proteins ubiquitously distributed in organisms, occurs as an individual protein in most archaeal phyla and has a structural similarity to Csp proteins, yet its biological functions remain unknown. Through physiological and biochemical studies on four TRAM proteins from a cold adaptive archaeon Methanolobus psychrophilus R15, this work demonstrated that TRAM is an archaeal Csp and exhibits RNA chaperone activity. Three TRAM encoding genes (Mpsy_0643, Mpsy_3043, and Mpsy_3066) exhibited remarkable cold-shock induced transcription and were preferentially translated at lower temperature (18°C), while the fourth (Mpsy_2002) was constitutively expressed. They were all able to complement the cspABGE mutant of Escherichia coli BX04 that does not grow in cold temperatures and showed transcriptional antitermination. TRAM3066 (gene product of Mpsy_3066) and TRAM2002 (gene product of Mpsy_2002) displayed sequence-non-specific RNA but not DNA binding activity, and TRAM3066 assisted RNases in degradation of structured RNA, thus validating the RNA chaperone activity of TRAMs. Given the chaperone activity, TRAM is predicted to function beyond a Csp.

Architecture and conservation of the bacterial DNA replication machinery, an underexploited drug target

New antibiotics with novel modes of action are required to combat the growing threat posed by multi-drug resistant bacteria. Over the last decade, genome sequencing and other high-throughput techniques have provided tremendous insight into the molecular processes underlying cellular functions in a wide range of bacterial species. We can now use these data to assess the degree of conservation of certain aspects of bacterial physiology, to help choose the best cellular targets for development of new broad-spectrum antibacterials.

DNA replication is a conserved and essential process, and the large number of proteins that interact to replicate DNA in bacteria are distinct from those in eukaryotes and archaea; yet none of the antibiotics in current clinical use acts directly on the replication machinery. Bacterial DNA synthesis thus appears to be an underexploited drug target. However, before this system can be targeted for drug design, it is important to understand which parts are conserved and which are not, as this will have implications for the spectrum of activity of any new inhibitors against bacterial species, as well as the potential for development of drug resistance. In this review we assess similarities and differences in replication components and mechanisms across the bacteria, highlight current progress towards the discovery of novel replication inhibitors, and suggest those aspects of the replication machinery that have the greatest potential as drug targets.