G-quadruplexes (G4s) are noncanonical nucleic acid secondary structures formed by guanine-rich DNA and RNA sequences. In this review we aim to provide an overview of the biological roles of G4s in microbial genomes with emphasis on recent discoveries. G4s are enriched and conserved in the regulatory regions of microbes, including bacteria, fungi, and viruses. Importantly, G4s in hepatitis B virus (HBV) and hepatitis C virus (HCV) genomes modulate genes crucial for virus replication. Recent studies on Epstein-Barr virus (EBV) shed light on the role of G4s within the microbial transcripts as cis-acting regulatory signals that modulate translation and facilitate immune evasion. Furthermore, G4s in microbial genomes have been linked to radioresistance, antigenic variation, recombination, and latency. G4s in microbial genomes represent novel therapeutic targets for antimicrobial therapy.

Depletion of CpG dinucleotides in SARS-CoV-2 genomes has been linked to virus evolution, host-switching, virus replication, and innate immune responses. Temporal variations, if any, in the rate of CpG depletion during virus evolution in the host remain poorly understood. Here, we analysed the CpG content of over 1.4 million full-length SARS-CoV-2 genomes representing over 170 million documented infections during the first 17 months of the pandemic. Our findings suggest that the extent of CpG depletion in SARS-CoV-2 genomes is modest. Interestingly, the rate of CpG depletion is highest during early evolution in humans and it gradually tapers off almost reaching an equilibrium; this is consistent with adaptations to the human host. Furthermore, within the coding regions, CpG depletion occurs predominantly at codon positions 2-3 and 3-1. Loss of ZAP-binding motifs in SARS-CoV-2 genomes is primarily driven by the loss of the terminal CpG in the motifs. Nonetheless, majority of the CpG depletion in SARS-CoV-2 genomes occurs outside ZAP-binding motifs. SARS-CoV-2 genomes selectively lose CpGs-motifs from a U-rich context; this may help avoid immune recognition by TLR7. SARS-CoV-2 alpha-, beta- and delta-variants of concern have reduced CpG content compared to sequences from the beginning of the pandemic. In sum, we provide evidence that the rate of CpG depletion in virus genomes is not uniform and it greatly varies over time and during adaptations to the host. This work highlights how temporal variations in selection pressures during virus adaption may impact the rate and the extent of CpG depletion in virus genomes.

Background Several lines of evidence suggest that recombination plays a central role in replication and evolution of herpes simplex virus-1 (HSV-1). G-quadruplex (G4)-motifs have been linked to recombination events in human and microbial genomes, but their role in recombination has not been studied in DNA viruses. Results The availability of near full-length sequences from 40 HSV-1 recombinant strains with exact position of the recombination breakpoints provided us with a unique opportunity to investigate the role of G4-motifs in recombination among herpes viruses. We mapped the G4-motifs in the parental and all the 40 recombinant strains. Interestingly, the genome-wide distribution of breakpoints closely mirrors the G4 densities in the HSV-1 genome; regions of the genome with higher G4 densities had higher number of recombination breakpoints. Biophysical characterization of oligonucleotides from a subset of predicted G4-motifs confirmed the formation of G-quadruplex structures. Our analysis also reveals that G4-motifs are enriched in regions flanking the recombination breakpoints. Interestingly, about 11% of breakpoints lie within a G4-motif, making these DNA secondary structures hotspots for recombination in the HSV-1 genome. Breakpoints within G4-motifs predominantly lie within G4-clusters rather than individual G4-motifs. Of note, we identified the terminal guanosine of G4-clusters at the boundaries of the U L (unique long) region on either side of the OriL (origin of replication within U L ) represented the commonest breakpoint among the HSV-1 recombinants. Conclusion Our findings suggest a correlation between the HSV-1 recombination landscape and the distribution of G4-motifs and G4-clusters, with possible implications for the evolution of DNA viruses. Electronic supplementary material The online version of this article (10.1186/s12864-019-5731-0) contains supplementary material, which is available to authorized users.