Project description:Directed evolution in mammalian cells can facilitate the engineering of mammalian-compatible biomolecules and can enable synthetic evolvability for mammalian cells. We engineered an orthogonal alphaviral RNA replication system to evolve synthetic RNA-based devices, enabling RNA replicase-assisted continuous evolution (REPLACE) in live mammalian cells. To investigate the process of mutation accumulation in REPLACE system, we constructed a repRNA-v4 plasmid library containing 64 barcodes. Using this library, we analyzed the differences in mutation accumulation for different RNAs upon entry into cells, before and after molnupiravir treatment, and before and after FACS sorting. The results demonstrated that these barcoded RNAs undergo similar processes of mutation accumulation, providing evidence that mutations are commonly accumulated across different RNAs.
Project description:Low copy number plasmids must encode maintenance mechanisms, such as partitioning systems, to ensure that the plasmid is sustained through host generations. Plasmid partition systems segregate sister plasmid copies and are subdivided into different types based on the NTPase they encode. The characterisation and distribution of partition system types is well understood in Enterobacteriaceae plasmids. However, how these systems maintain plasmids and are distributed across wider bacterial diversity is poorly understood. We used the Streptomyces coelicolor A3(2) plasmid SCP1, which encodes two type Ia partition systems, as a model to investigate this. Sequence analysis of the SCP1 partition systems revealed that both ParB proteins contain less conserved CTP-binding pockets, suggesting one or both proteins may not behave like canonical ParB proteins. However, using a combination of chromatin immunoprecipitation with deep sequencing (ChIP-seq) we demonstrate that both the SCP1 ParB proteins, ParB1 and ParB2, bound to distinct parS sites on SCP1, and accumulate, or spread, on DNA approximately 20 kb away from their initial parS loading site. Together, our findings further our understanding of Streptomyces plasmid maintenance by providing the first functional characterisation of two type Ia partition systems coexisting on a single plasmid and offer new insights into the diversity and distribution of plasmid partition systems.
Project description:Sequencing of oligonucleotide barcodes holds promise as a high-throughput approach for reconstructing synaptic connectivity at scale. Rabies viruses can act as a vehicle for barcode transmission, thanks to their ability to spread between synaptically connected cells. However, applying barcoded rabies viruses to map synaptic connections in vivo has proved challenging. Here, we develop Barcoded Rabies In Situ Connectomics (BRISC) for high-throughput connectivity mapping in the mouse brain. To ensure that the majority of post-synaptic "starter" neurons are uniquely labeled with distinct barcode sequences, we first generated libraries of rabies viruses with sufficient diversity to label >1000 neurons uniquely. To minimize the probability of barcode transmission between starter neurons, we developed a strategy to tightly control their density. We then applied BRISC to map inputs of single neurons in the primary visual cortex (V1). Using in situ sequencing, we read out the expression of viral barcodes in rabies-infected neurons, while preserving spatial information. We then matched barcode sequences between starter and presynaptic neurons, mapping the inputs of 385 neurons and identifying 7,814 putative synaptic connections. The resulting connectivity matrix revealed layer- and cell-type-specific local connectivity rules and topographic organization of long-range inputs to V1. These results show that BRISC can simultaneously resolve the synaptic connectivity of hundreds of neurons while preserving spatial information, enabling reconstruction of neural circuits at an unprecedented scale.
Project description:Low copy number plasmids must encode maintenance mechanisms, such as partitioning systems, to ensure that the plasmid is sustained through host generations. Plasmid partition systems segregate sister plasmid copies and are subdivided into different types based on the NTPase they encode. The characterisation and distribution of partition system types is well understood in Enterobacteriaceae plasmids. However, how these systems maintain plasmids and are distributed across wider bacterial diversity is poorly understood. We searched a large and diverse plasmid database to identify the distribution of partition system types and found that plasmids encoding multiple partition systems are more widespread than previously realised, composing ~19% of all plasmids predicted to encode partition systems. Many of these plasmids were predicted to encode multiple partition systems of the same type, which had never been studied previously. We used the Streptomyces coelicolor A3(2) plasmid SCP1, which encodes two type Ia partition systems, as a model to investigate this. Sequence analysis of the SCP1 partition systems revealed that both ParB proteins contain less conserved CTP-binding pockets, suggesting one or both proteins may not behave like canonical ParB proteins. However, using a combination of chromatin immunoprecipitation with deep sequencing (ChIP-seq), biochemistry and mutagenesis we show that both ParB proteins behave like bone fide ParB proteins: they bound to distinct parS sites on SCP1, both proteins bound and hydrolysed CTP and both proteins required CTP and their cognate parS site to accumulate, or spread, on DNA. Intriguingly, however, only ParB1 was critical for SCP1 stability. Together, our findings further our understanding of Streptomyces plasmid maintenance by providing the first functional characterisation of two type Ia partition systems coexisting on a single plasmid and offer new insights into the diversity and distribution of plasmid partition systems.