Characterizing the transcriptome of individual cells is fundamental to understanding complex biological systems. We describe a droplet-based system that enables 3′ mRNA counting of tens of thousands of single cells per sample. Cell encapsulation, of up to 8 samples at a time, takes place in ∼6 min, with ∼50% cell capture efficiency. To demonstrate the system's technical performance, we collected transcriptome data from ∼250k single cells across 29 samples. We validated the sensitivity of the system and its ability to detect rare populations using cell lines and synthetic RNAs. We profiled 68k peripheral blood mononuclear cells to demonstrate the system's ability to characterize large immune populations. Finally, we used sequence variation in the transcriptome data to determine host and donor chimerism at single-cell resolution from bone marrow mononuclear cells isolated from transplant patients.

40Characterizing the transcriptome of individual cells is fundamental to understanding complex 41 biological systems. We describe a droplet-based system that enables 3' mRNA counting of up 56peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/065912 doi: bioRxiv preprint first posted online 84 RESULTS 86Droplet-based platform enables barcoding of tens of thousands of cells 88The scRNA-seq microfluidics platform builds on the GemCode ® technology, which has 89 been used for genome haplotyping, structural variant analysis and de novo assembly of a 90human genome [10][11][12] . The core of the technology is a Gel bead in Emulsion (GEM). GEM 91 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/065912 doi: bioRxiv preprint first posted online 5 generation takes place in an 8-channel microfluidic chip that encapsulates single gel beads at ~80% fill rate (Fig. 1a-c). Each gel bead is functionalized with barcoded oligonucleotides that 93 consist of: i) sequencing adapters and primers, ii) a 14bp barcode drawn from ~750,000 94 designed sequences to index GEMs, iii) a 10bp randomer to index molecules (unique molecular 95 identifier, UMI), and iv) an anchored 30bp oligo-dT to prime poly-adenylated RNA transcripts 96 (Fig. 1d). Within each microfluidic channel, ~100,000 GEMs are formed per ~6-min run, 97encapsulating thousands of cells in GEMs. Cells are loaded at a limiting dilution to minimize co- 98occurrence of multiple cells in the same GEM. 100Cell lysis begins immediately after encapsulation. Gel beads automatically dissolve to 101 release their oligonucleotides for reverse transcription of poly-adenylated RNAs. Each cDNA 102 molecule contains a UMI and shared barcode per GEM, and ends with a template switching 103 oligo at the 3' end (Fig. 1e). Next, the emulsion is broken and barcoded cDNA is pooled for 104PCR amplification, using primers complementary to the switch oligos and sequencing adapters. Methods, Fig. 1f). Briefly, 98-nt of Read1s were aligned against the union of human (hg19) 123peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/065912 doi: bioRxiv preprint first posted online 6 Based on the distribution of total UMI counts for each barcode (Online Methods), we 124 estimated that 1,012 GEMs contained cells, of which 482 and 538 contained reads that mapped 125 primarily to the human and mouse transcriptome, respectively (and will be referred to as human 126 and mouse GEMs) (Fig. 2a). >83% of UMI counts were associated with cell barcodes, 127indicating low background of cell-free RNA. Eight cell-containing GEMs had a substantial 128 fraction of human and mouse UMI counts (the UMI count is >1% of each species' UMI...

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DNA polymerase ζ (Pol ζ) is a eukaryotic B-family DNA polymerase that specializes in translesion synthesis and is essential for normal embryogenesis. At a minimum, Pol ζ consists of a catalytic subunit Rev3 and an accessory subunit Rev7. Mammalian Rev3 contains >3,000 residues and is twice as large as the yeast homolog. To date, no vertebrate Pol ζ has been purified for biochemical characterization. Here we report purification of a series of human Rev3 deletion constructs expressed in HEK293 cells and identification of a minimally catalytically active human Pol ζ variant. With a tagged form of an active Pol ζ variant, we isolated two additional accessory subunits of human Pol ζ, PolD2 and PolD3. The purified foursubunit Pol ζ4 (Rev3-Rev7-PolD2-PolD3) is much more efficient and more processive at bypassing a 1,2-intrastrand d(GpG)-cisplatin cross-link than the two-subunit Pol ζ2 (Rev3-Rev7). We show that complete bypass of cisplatin lesions requires Pol η to insert dCTP opposite the 3′ guanine and Pol ζ4 to extend the primers., composed of the catalytic Rev3 and accessary Rev7 subunits, is an error-prone DNA translesion polymerase that causes both spontaneous and DNA damage-induced mutagenesis (1, 2). More than two-thirds of the 1,504 residues in yeast Rev3 share sequence homology with all B-family DNA polymerases, including Pols α, δ, and e, which are responsible for the bulk of high-fidelity genomic replication in eukaryotes (3). Unlike the typical B-family polymerases, Pol ζ lacks an intrinsic 3′-5′ exonuclease activity and thus has no proofreading function (2). Human homologs of REV3 (REV3L) and REV7 (MAD2L2; hereafter referred to as REV7) genes were identified shortly after yeast Pol ζ was characterized. Human Rev3 contains 3,130 residues and is twice as large as the yeast counterpart (4). Human and yeast Rev7 are homologous (5) and bear sequence similarity to the mitotic checkpoint proteins Mad2 (6). Unlike Saccharomyces cerevisiae REV3 and REV7 genes, which are nonessential and whose knockout leads only to a decreased rate of damage-induced mutagenesis (7, 8), Rev3l knockout in mice is embryonic-lethal (9), and mouse Rev3l −/− embryonic stem cells are not viable (10, 11). Human and mouse cell cultures obtained from conditional Rev3l knockout show genome instability and growth defects without an external challenge of DNA damage (12)(13)(14). DNA pol ζ is apparently essential for normal cell proliferation and embryogenesis in mammals.Translesion synthesis (TLS) and DNA-damage-induced mutagenesis are the best-characterized functions of Pol ζ. Absence of the yeast REV3 gene leads to sensitivity to UV light and intrastrand and interstrand cross-linking agents (2, 15). DNA Pol ζ has been shown to induce multiple base substitutions as well as more complex mutations in yeast (7,16,17) and may contribute to hypermutation in Ig genes in mammals (18,19). The TLS function of DNA Pol ζ has been implicated in its role of mediating resistance to platinum-based chemotherapies (20)(21)(22). Owing to the conservation of B-f...