Drosophila Cas9 Expression pUASTattB Vector (UAS-Hsp70 promoter）
Our Drosophila Cas9 expression pUASTattB vector is a highly effective system for generating transgenic flies with conditional Cas9 protein expression. This vector system combines Cas9 expression for CRISPR gene editing and bacteriophage φC31 integrase-mediated recombination, and a strong Gal4-inducible promoter to regulate Cas9 protein expression. .
The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system has greatly facilitated inactivation of genes in vitro and in vivo in a wide range of organisms. In this genome-editing system, the Cas9 enzyme forms a complex with a guide RNA (gRNA), which provides targeting specificity through direct interaction with homologous 18-22nt target sequences in the genome. Hybridization of the gRNA to the target site localizes Cas9, which then cuts the target site in the genome. Cas9 screens the genome and cleaves within sequences complementary to the gRNA, provided they are immediately followed by the protospacer adjacent motif (PAM) NGG. Double strand breaks are then repaired via homologous recombination or non-homologous end-joining, resulting in indels (insertion or deletion of bases in the genome) of variable length. Utilizing the CRISPR/Cas9 system in Drosophila allows the rapid generation of knockout lines by simply delivering either an all-in-one vector (a single vector expressing both Cas9 and gRNA) or separate vectors for driving Cas9 and gRNA expression, respectively.
This pUASTattB system consists of three primary elements: (1) φC31 integrase-mediated insertion elements attB and attP, (2) GAL4 binding sites for selective gene expression, and (3) a heat-shock inducible (hsp70) promoter for Cas9 expression.
The attB vector system consists of two vectors, both engineered as E.coli plasmids. One vector referred to as the attB vector or the φC31 donor vector carries the attB site and gene of interest (Cas9). The other vector referred to as the helper plasmid encodes the φC31 integrase. When the attB and φC31 helper plasmids are co-injected into cells containing attP landing sites, φC31 integrase mediates recombination between attB and attP sites, resulting in the linearization and integration of the attB vector into the host genome. Alternatively, the donor vector can be injected into cells from a Drosophila φC31 integrase-expressing line.
The bacteriophage φC31 encodes an integrase that mediates efficient, sequence-specific recombination between phage attachment sites (called attP) and bacterial attachment sites (called attB). In contrast to transposon-based systems, such as P-element-mediated transposition, φC31-mediated insertion is irreversible. Integration of attB into an attP position creates hybrid sites (called attL and attR), which are refractory to the φC31 integrase. Additionally, φC31-based insertion is site-specific, generally occurring only at attP sites, and not elsewhere in the genome. For this reason, the attB vector system is designed to be used with Drosophila lines carrying attP “landing sites” within their genome.
This GAL4/UAS system is designed to direct selective, GAL4-dependent expression of the Cas9 gene. The GAL4 protein activates transcription upon binding to the UAS sites upstream of the Cas9 gene. Therefore, in the absence of GAL4 expression the Cas9 gene remains silent, but introduction of GAL4 by crossing to a GAL4-expressing Drosophila line results in transcriptional activation.
In this Cas9 expression pUASTattB vector, a Cas9 gene is cloned downstream of an engineered, inducible promoter consisting of five tandemly arrayed GAL4 binding sites (5xUAS) and the heat shock protein hsp70 TATA box promoter. Incubation at 37℃ activates the promoter and subsequent Cas9 expression. Additionally, the mini white gene on the pUASTattB vector encodes eye color and acts as a marker for the identification of transgenic flies which have undergone successful recombination of the transgene. PCR or other molecular methods can also be used to identify transgenic cells or animals.
For further information about this vector system, please refer to the papers below.
|Methods Mol Biol. 420:61 (2008)||The use of P-element transposons to generate transgenic flies|
|Proc Natl Acad Sci U S A. 97:5995 (2000)|
Proc Natl Acad Sci U S A. 97:5995 (1998)
|Description of the φC31 integrase system|
|Proc Natl Acad Sci U S A. 104:3312-7 (2007)||Generation of φC31-based transgenic Drosophila|
|Science. 339:819-23 (2013)||Description of genome editing using the CRISPR/Cas9 system|
|Methods Mol Biol. 2540:135-156 (2022)||CRISPR-mediated genome editing in Drosophila|
Our Drosophila Cas9 expression pUASTattB vector is a highly effective system for generating φC31 integrase-mediated Cas9 gene insertion and selective GAL4-dependent expression of Cas9 protein. Our vectors are optimized for high copy number replication in E. coli and high-efficiency transgenesis of Drosophila lines.
Site-specific insertion: φC31-based insertion is site-specific, generally occurring only at attP sites. This reduces the risk of disrupting endogenous genes or having insertion site position that affects transgene expression.
High-level expression: The 5×UAS/mini_Hsp70 promoter can drive strong expression of the gene of interest in its induced state.
Selective expression: In the absence of GAL4, transcription of the gene of interest should be very low or silent, while in the presence of GAL4, high level of gene transcription is achieved.
High efficiency: Achieving germ-line transgenesis using φC31 integrase vectors is more efficient than P-element based systems such as pUAST.
Potentially leaky expression: In some cases, low-level expression of the gene of interest can occur in the absence of GAL4.
Technical complexity: The generation of transgenic Drosophila requires embryonic injection and fly husbandry, which can be technically difficult.
Requires attP insertion site: The generation of transgenic Drosophila using the pUASTattB vector requires the use of specialized host lines carrying attP “landing sites” in their genome.
5×UAS/mini_Hsp70: The Drosophila melanogaster heat shock protein 70 (Hsp70) minimal promoter fused with five tandem galactose upstream activating sequences (5×UAS). This is a strong promoter, tightly inducible by GAL4.
Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest because it is believed to facilitate translation initiation in eukaryotes.
Cas9: a CRISPR-associated endonuclease that cuts DNA at a location specified by gRNA.
SV40 terminator: Simian virus 40 transcriptional terminator. Contains the SV40 small T intron and the SV40 early polyadenylation signal.
attB site: The bacterial attachment site, attB, recognized by the bacteriophage φC31 serine integrase. φC31 integrase can catalyze site-specific integration of attB-containing plasmids into attP-containing docking or landing sites that have been introduced into host genomes.
pUC ori: pUC origin of replication. Plasmids carrying this origin exist in high copy numbers in E. coli.
Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.
mini-white: A variant of the Drosophila white gene. The mini-white gene is a dominant marker for adult fruit fly eye color, which can be used as a reporter to identify transgenic events in a white mutant background.