Shilton, A. | bioRxiv Science

bioRxiv2024

The Cas10 nuclease activity relieves host dormancy to facilitate spacer acquisition and retention during type III-A CRISPR immunity

A hallmark of CRISPR immunity is the acquisition of short viral DNA sequences, known as spacers, that are transcribed into guide RNAs to recognize complementary sequences. The staphylococcal type III-A CRISPR-Cas system uses guide RNAs to locate viral transcripts and start a response that displays two mechanisms of immunity. When immunity is triggered by an early-expressed phage RNA, degradation of viral ssDNA can cure the host from infection. In contrast, when the RNA guide targets a late-expressed transcript, defense requires the activity of Csm6, a non-specific RNase. Here we show that Csm6 triggers a growth arrest of the host that provides immunity at the population level which hinders viral propagation to allow the replication of non-infected cells. We demonstrate that this mechanism leads to defense against not only the target phage but also other viruses present in the population that fail to replicate in the arrested cells. On the other hand, dormancy limits the acquisition and retention of spacers that trigger it. We found that the ssDNase activity of type III-A systems is required for the re-growth of a subset of the arrested cells, presumably through the degradation of the phage DNA, ending target transcription and inactivating the immune response. Altogether, our work reveals a built-in mechanism within type III-A CRISPR-Cas systems that allows the exit from dormancy needed for the subsistence of spacers that provide broad-spectrum immunity.

microbiology↗

bioRxiv2020

Origin and evolution of the Zinc Finger Antiviral Protein

The human zinc finger antiviral protein (ZAP) recognizes RNA by binding to CpG dinucleotides. Mammalian transcriptomes are CpG-poor, and ZAP may have evolved to exploit this feature to specifically target non-self viral RNA. Phylogenetic analyses reveal that ZAP and its paralogue PARP12 share an ancestral gene that arose prior to extensive eukaryote divergence, and the ZAP lineage diverged from the PARP12 lineage in tetrapods. Notably, The CpG content of modern eukaryote genomes varies widely, and ZAP-like genes arose subsequent to the emergence of CpG-suppression in vertebrates. Human PARP12 exhibited no antiviral activity against wild type and CpG-enriched HIV-1, but ZAP proteins from several tetrapods had antiviral activity when expressed human cells. In some cases, ZAP antiviral activity required a TRIM25 protein from the same or a related species, suggesting functional co-evolution of these genes. Indeed, a hypervariable sequence in the N-terminal domain of ZAP contributed to species-specific TRIM25 dependence in antiviral activity assays. Crosslinking immunoprecipitation coupled with RNA sequencing revealed that ZAP proteins from human, mouse, bat and alligator exhibit a high degree of CpG-specificity, while some avian ZAP proteins appear more promiscuous. Together, these data suggest that the CpG-rich RNA directed antiviral activity of ZAP-related proteins arose in tetrapods, subsequent to the onset of CpG suppression in certain eukaryote lineages, with subsequent species-specific adaptation of cofactor requirements and RNA target specificity. Author SummaryTo control viral infections, cells have evolved a variety of mechanisms that detect, modify and sometimes eliminate viral components. One of such mechanism is the Zinc Finger Antiviral Protein (ZAP) which binds RNA sequences that are rich in elements composed of a cytosine followed by a guanine. Selection of viral RNA can only be achieved because such elements are sparse in RNAs encoded by human genes. Here, we traced the molecular evolution of ZAP. We found that ZAP and a closely related gene, PARP12, originated from the same ancestral gene that existed in a predecessor of vertebrates and invertebrates. We found that ZAP proteins from mammals, birds and reptiles have antiviral activity but only in the presence of a co-factor, TRIM25, from the same species. ZAP proteins from birds were particularly interesting since they demonstrated a broader antiviral activity, primarily driven by relaxed requirement for cytosine-guanine. Our findings suggest that viruses that infect birds - which are important vectors for human diseases - are under differential selective pressures and this property may influence the outcome of interspecies transmission.

microbiology↗