10A growing number of CRISPR-Cas9 associated applications require co-expression of two distinct 11 gRNAs. However, coexpressing paired gRNAs under the driving of independent but identical 12 promoters in the same direction triggers plasmid instability, due to the presence of direct repeats 13 (DRs). In this study, deletion between DRs occurred with high frequencies during plasmid 14 construction and duplication processes, when three DRs-involved paired-gRNA plasmids cloning 15 strategies were tested. This recombination phenomenon was RecA-independent, in agreement with 16 the replication slippage model. To completely eliminate the DRs-induced plasmid instability, a 17 reversed paired-gRNA plasmids (RPGPs) cloning strategy was developed by converting DRs to the 18 more stable invert repeats (IRs). Using RPGPs, we achieved a rapid deletion of chromosome 19 fragments up to 100 kb with high efficiency of 83.33% in Escherichia coli. This study provides 20 general solutions to construct stable plasmids containing short DRs, which can improve the 21 performances of CRISPR systems that relied on paired gRNAs, and also facilitate other applications 22 involving repeated genetic parts. 23 24 Keywords 25 paired gRNAs; plasmid stability; direct repeats; inverted repeats; large genomic deletion 26 27 28 3 / 20To test whether the recombination of pDG-A-X series relied on the RecA enzyme, the correct pDG-111 A-100K was re-transformed into various E. coli strains with the genotypes of recA1, ΔrecA1398, 112 Δ(sr1-recA) or recA + (Figure 1E). In MG1655 recA + strain expressing functional RecA protein, the 113 deletion rate was up to 91.67%. In various recA mutant strains, DRs-induced recombination still 114 occurred with the frequencies of 63.33-95%. No distinct difference of deletion rates was found in the other one is the repeated 82-bp gRNA scaffold. To reduce the number of DRs, pDG-P-X was 132 designed by replacing the second promoter J23119 with an alternative 49-bp PR promoter ( Figure 133 4A). After the assembly products of pDG-P-100K were introduced into DH10B strain, the deletion 134 rate of pDG-P-X was up to 81.67% when verified by primers F1 /R1 (Figure 4B). These deletion 135 derivatives of pDG-P-100K didn't contain promoter PR region when verified by primers F2/R1. 136 DNA sequencing demonstrated that pDG-P-100K generated spontaneous deletion of the second 137 gRNA region to form pDG-A-100K-M2. Although it was difficult to obtain correct pDG-P-X series in multiple mechanisms, Proc Natl Acad Sci USA 98: 8319-8325. 339