CRISPR-Based Screening of Genomic Island Excision Events in Bacteria
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CRISPR-based screening of genomic island excision events in bacteria Kurt Sellea,b, Todd R. Klaenhammera,b,1, and Rodolphe Barrangoua,1 aDepartment of Food, Bioprocessing and Nutrition Sciences, North Carolina State University, Raleigh, NC 27695; and bFunctional Genomics Graduate Program, North Carolina State University, Raleigh, NC 27695 Contributed by Todd R. Klaenhammer, May 19, 2015 (sent for review September 22, 2014) Genomic analysis of Streptococcus thermophilus revealed that RNAs (crRNAs) that guide Cas proteins for sequence-specific mobile genetic elements (MGEs) likely contributed to gene acqui- recognition and cleavage of target DNA complementary to the sition and loss during evolutionary adaptation to milk. Clustered spacer (7). CRISPR-Cas technology has applications in strain regularly interspaced short palindromic repeats–CRISPR-associated typing and detection (8–10), exploitation of natural/engineered genes (CRISPR-Cas), the adaptive immune system in bacteria, limits immunity against mobile genetic elements (11), programmable genetic diversity by targeting MGEs including bacteriophages, genome editing in diverse backgrounds (12), transcriptional con- transposons, and plasmids. CRISPR-Cas systems are widespread trol (13, 14), and manipulation of microbial populations in defined in streptococci, suggesting that the interplay between CRISPR- consortia (15). Cas systems and MGEs is one of the driving forces governing Although sequence features corresponding to CRISPR arrays genome homeostasis in this genus. To investigate the genetic out- were described previously in multiple organisms (16, 17), Strep- in comes resulting from CRISPR-Cas targeting of integrated MGEs, tococcus thermophilus was the first microbe in which the roles silico prediction revealed four genomic islands without essential of specific cas genes and CRISPR-array components were elu- genes in lengths from 8 to 102 kbp, totaling 7% of the genome. cidated (4). S. thermophilus is a nonpathogenic, thermophilic In this study, the endogenous CRISPR3 type II system was pro- Gram-positive bacterium used as a starter culture that catabo- grammed to target the four islands independently through plas- lizes lactose to lactic acid in the syntrophic production of yogurt mid-based expression of engineered CRISPR arrays. Targeting lacZ and various cheeses (18). S. thermophilus encodes up to four within the largest 102-kbp genomic island was lethal to wild-type CRISPR-Cas systems, two of which (CRISPR1 and CRISPR3) cells and resulted in a reduction of up to 2.5-log in the surviving population. Genotyping of Lac− survivors revealed variable dele- are classified as type II-A systems that are innately active in both acquisition and interference (4, 19). Accordingly, genomic tion events between the flanking insertion-sequence elements, all S thermophilus resulting in elimination of the Lac-encoding island. Chimeric inser- analysis of . and its bacteriophages established a tion sequence footprints were observed at the deletion junctions likely mechanism for phage/DNA protection in CRISPR-Cas S thermophilus after targeting all of the four genomic islands, suggesting a com- systems. Investigation of CRISPR-Cas systems in . mon mechanism of deletion via recombination between flanking led to bioinformatic analysis of spacer origin (4, 20), discovery of insertion sequences. These results established that self-targeting the proto-spacer adjacent motif (PAM) sequences (19, 21), un- CRISPR-Cas systems may direct significant evolution of bacterial derstanding of phage–host dynamics (22, 23), demonstration genomes on a population level, influencing genome homeostasis of Cas9 endonuclease activity (7, 24, 25), and, recently, deter- and remodeling. mination of the transactivating crRNA tracrRNA structural motifs governing function and orthogonality of type II systems CRISPR | lactic acid bacteria | transposons | IS-elements | Cas9 (26). Genomic analysis of S. thermophilus revealed evolutionary obile genetic elements (MGEs) present bacteria with Significance Mcontinuous challenges to genomic stability, promoting evolution through horizontal gene transfer. The term “MGE” The development of Clustered regularly interspaced short pal- encompasses plasmids, bacteriophages, transposable elements, indromic repeats (CRISPR)–CRISPR-associated genes (CAS)–based genomic islands, and many other specialized genetic elements technology for targeted genome editing has revolutionized (1). MGEs encompass genes conferring high rates of dissemi- molecular biology approaches, but significant and outstanding nation, adaptive advantages to the host, and genomic stability, gaps exist for applications in bacteria, the native hosts of these leading to their nearly universal presence in bacterial genomes. adaptive immune systems. This study shows that CRISPR-Cas To cope with the permanent threat of predatory bacteriophages systems can be directed to target and delete genomic islands and selfish genetic elements, bacteria have evolved both innate that are flanked by insertion-sequence elements and devoid of and adaptive immune systems targeting exogenous genetic ele- essential genes. Naturally occurring minor subpopulations har- ments. Innate immunity includes cell-wall modification, restriction/ boring deletions in genomic islands were identified and readily modification systems, and abortive phage infection (2). Clustered isolated using CRISPR-Cas screening. Promising applications of regularly interspaced short palindromic repeats (CRISPR) and this approach can define minimal bacterial genomes, determine CRISPR-associated genes (Cas) are an adaptive immune system essential genes, and characterize genetically heterogeneous targeted against invasive genetic elements in bacteria (3). CRISPR- bacterial populations. Cas–mediated immunity relies on distinct molecular processes, Author contributions: K.S., T.R.K., and R.B. designed research; K.S. performed research; categorized as acquisition, expression, and interference (3). Ac- K.S. and T.R.K. contributed new reagents/analytic tools; K.S., T.R.K., and R.B. analyzed quisition occurs via molecular sampling of foreign genetic data; and K.S., T.R.K., and R.B. wrote the paper. “ ” elements, from which short sequences, termed spacers, are in- The authors declare no conflict of interest. tegrated in a polarized fashion into the CRISPR array (4). Freely available online through the PNAS open access option. Expression of CRISPR arrays is constitutive and inducible by 1To whom correspondence may be addressed. Email: [email protected] or promoter elements within the preceding leader sequence (5, 6). [email protected]. Interference results from a corresponding transcript that is This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. processed selectively at each repeat sequence, forming CRISPR 1073/pnas.1508525112/-/DCSupplemental. 8076–8081 | PNAS | June 30, 2015 | vol. 112 | no. 26 www.pnas.org/cgi/doi/10.1073/pnas.1508525112 Downloaded by guest on September 28, 2021 adaptation to milk through the loss of carbohydrate catabolism of related IS elements were mapped to the S. thermophilus genome and virulence genes found in pathogenic streptococci (18). (Fig. 1). The IS1193 and Sth6 families of IS elements appeared S. thermophilus also underwent significant acquisition of niche- most frequently in the genome and are commonly found in related genes, such as those encoding including cold-shock pro- Streptococcus pneumoniae and Streptococcus mutans (34). IS1191 teins, copper resistance proteins, proteinases, bacteriocins, and elements were not frequent but exhibited nearly perfect identity lactose catabolism proteins (18). Insertion sequences (ISs) are between the copies identified in the genome (Figs. S1A and highly prevalent in S. thermophilus genomes and contribute to S2A). Despite the prevalence of IS1193 elements, many of these genetic heterogeneity among strains by facilitating dissemination loci were shown to be small fragments that exhibited some of islands associated with dairy adaptation genes (18). The con- polymorphism and degeneracy, but several copies with a high comitant presence of MGEs and functional CRISPR-Cas systems level of sequence identity were present also (Figs. S1B and S2B). in S. thermophilus suggests that genome homeostasis is governed The Sth6 family exhibited considerable polymorphism and high at least in part by the interplay of these dynamic forces. Thus, degeneracy, with some copies harboring significant internal de- S. thermophilus constitutes an ideal host for investigating the genetic letions (Figs. S1C and S2C). IS1167 elements were well con- outcomes of CRISPR-Cas targeting of genomic islands. served (Figs. 1D and S2D). Based on the conservation of length CRISPR-Cas systems recently have been the subject of intense and sequence of the IS1167 and IS1191 elements of S. thermo- research in genome editing applications (12), but the evolutionary philus and their relative proximity to milk adaptation genes, we roles of most endogenous microbial systems remain unknown postulate that these conserved/high-fidelity transposons were (27). Even less is known concerning evolutionary outcomes of acquired in the genome recently. housing active CRISPR-Cas systems beyond the prevention of By combining the location of predicted essential ORFs and foreign DNA uptake (7), spacer acquisition events (4), and mu- highly similar IS elements, expendable islands were identified tation caused by chromosomal self-targeting