Plant epidermal cells express unique molecular machinery that juxtapose the assembly of intracellular lipid components and the unique extracellular cuticular lipids that are unidirectionally secreted to plant surfaces. In maize (Zea mays), mutations at the glossy2 (gl2) locus affect the deposition of extracellular cuticular lipids. Sequence-based genome scanning identified a new Gl2 homolog in the maize genome, namely Gl2-like. Both the Gl2-like and Gl2 genes are members of the BAHD superfamily of acyltransferases, with close sequence similarity to the Arabidopsis (Arabidopsis thaliana) CER2 gene. Transgenic experiments demonstrated that Gl2-like and Gl2 functionally complement the Arabidopsis cer2 mutation, with differential influences on the cuticular lipids and the lipidome of the plant, particularly affecting the longer alkyl chain acyl lipids, especially at the 32-carbon chain length. Site-directed mutagenesis of the putative BAHD catalytic HXXXDX-motif indicated that Gl2-like requires this catalytic capability to fully complement the cer2 function, but Gl2 can accomplish complementation without the need for this catalytic motif. These findings demonstrate that Gl2 and Gl2-like overlap in their cuticular lipid function, but have evolutionarily diverged to acquire nonoverlapping functions.

CRISPR-mediated interference relies on complementarity between a guiding CRISPR RNA (crRNA) and target nucleic acids to provide defense against bacteriophage. Phages escape CRISPR-based immunity mainly through mutations in the PAM and seed regions. However, previous specificity studies of Cas effectors, including the class 2 endonuclease Cas12a, have revealed a high degree of tolerance of single mismatches. The effect of this mismatch tolerance has not been extensively studied in the context of phage defense. Here, we tested defense against lambda phage provided by Cas12a-crRNAs containing pre-existing mismatches against the genomic targets in phage DNA. We observe a correlation between Cas12a mismatch tolerance in vitro and phage defense on solid media. However, in liquid media, we find that most pre-existing crRNA mismatches lead to phage escape and lysis, regardless of whether the mismatches ablate Cas12a cleavage in vitro. We used high-throughput sequencing to examine the target regions of phage genomes following CRISPR challenge. Mismatches at all locations in the target accelerated emergence of mutant phage, including mismatches that greatly slowed cleavage in vitro. Mutations arose near the existing mismatch, in some cases resulting in multiple PAM-distal mismatches allowing for phage escape. Similar experiments with Cas9 showed the location of emergent target mutations was unaffected by pre-existing crRNA-target mismatches. Expression of multiple mismatched crRNAs prevented new mutations from arising in multiple targeted locations, allowing Cas12a mismatch tolerance to provide stronger and longer term protection. These results demonstrate that Cas effector mismatch tolerance and existing target mismatches strongly influence phage evolution.

CRISPR-mediated interference relies on complementarity between a guiding CRISPR RNA (crRNA) and target nucleic acids to provide defense against bacteriophage. Phages escape CRISPR-based immunity mainly through mutations in the protospacer adjacent motif (PAM) and seed regions. However, previous specificity studies of Cas effectors, including the class 2 endonuclease Cas12a, have revealed a high degree of tolerance of single mismatches. The effect of this mismatch tolerance has not been extensively studied in the context of phage defense. Here, we tested defense against lambda phage provided by Cas12a-crRNAs containing preexisting mismatches against the genomic targets in phage DNA. We find that most preexisting crRNA mismatches lead to phage escape, regardless of whether the mismatches ablate Cas12a cleavage in vitro. We used high-throughput sequencing to examine the target regions of phage genomes following CRISPR challenge. Mismatches at all locations in the target accelerated emergence of mutant phage, including mismatches that greatly slowed cleavage in vitro. Unexpectedly, our results reveal that a preexisting mismatch in the PAM-distal region results in selection of mutations in the PAM-distal region of the target. In vitro cleavage and phage competition assays show that dual PAM-distal mismatches are significantly more deleterious than combinations of seed and PAM-distal mismatches, resulting in this selection. However, similar experiments with Cas9 did not result in emergence of PAM-distal mismatches, suggesting that cut-site location and subsequent DNA repair may influence the location of escape mutations within target regions. Expression of multiple mismatched crRNAs prevented new mutations from arising in multiple targeted locations, allowing Cas12a mismatch tolerance to provide stronger and longer-term protection. These results demonstrate that Cas effector mismatch tolerance, existing target mismatches, and cleavage site strongly influence phage evolution.

Type I, II and V CRISPR-Cas systems are RNA-guided dsDNA targeting defense mechanisms found in bacteria and archaea. During CRISPR interference, Cas effectors use CRISPR-derived RNAs (crRNAs) as guides to bind complementary sequences in foreign dsDNA, leading to the cleavage and destruction of the DNA target. Mutations within the target or in the protospacer adjacent motif (PAM) can reduce the level of CRISPR interference, although the level of defect is dependent on the type and position of the mutation, as well as the guide sequence of the crRNA. Given the importance of Cas effectors in host defense and for biotechnology tools, there has been considerable interest in developing sensitive methods for detecting Cas effector activity through CRISPR interference. In this chapter, we describe an in vivo fluorescence-based method for monitoring plasmid interference in Escherichia coli. This approach uses a green fluorescent protein (GFP) reporter to monitor varying plasmid levels within bacterial colonies, or to measure the rate of plasmid loss in bacterial populations over time. We demonstrate the use of this simple plasmid loss assay for both chromosomally integrated and plasmid-borne CRISPR-Cas systems.

51Plant epidermal cells express unique molecular machinery that juxtapose the assembly of 52 intracellular lipid components and the unique extracellular cuticular lipids that are 53 unidirectionally secreted to plant surfaces. In maize (Zea mays L.), mutations at the glossy2 (gl2) 54 locus affect the deposition of extracellular cuticular lipids. Sequence-based genome scanning 55 identified a novel gl2 homolog in the maize genome, Gl2-like. Sequence homology identifies 56 that both the Gl2-like and Gl2 genes are members of the BAHD superfamily of acyltransferases, 57 with close sequence homology to the Arabidopsis CER2 gene. Transgenic experiments 58 demonstrate that Gl2-like and Gl2 functionally complement the Arabidopsis cer2 mutation, with 59 differential impacts on the cuticular lipids and the lipidome of the plant, particularly affecting the 60 longer alkyl chain acyl lipids, particularly at the 32-carbon chain length. Site-directed 61 mutagenesis of the putative BAHD catalytic HXXXDX-motif indicates that Gl2-like requires 62 this catalytic capability to fully complement the cer2 function, but Gl2 can accomplish this 63 without the need for this catalytic motif. These findings demonstrate that both Gl2 and Gl2-like 64 overlap in their cuticular lipid function, however the two genes have evolutionary diverged to 65 acquire non-overlapping functions. 66 67