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. It also incorporates a user-defined promoter to achieve ubiquitous, tissue-specific or inducible 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 two primary elements: (1) φC31 integrase-mediated insertion elements attB and attP and (2) a user-defined promoter upstream of the GOI (Cas9). 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 Cas9 expression pUASTattB vector allows users to paste the sequence for their chosen promoter or select a promoter of their choice from our Drosophila promoter database, depending on their experimental goal. Users have the choice of the following promoters: ubiquitous promoters including actin 5C, polyubiquitin, and alpha-1 tubulin; tissue-specific promoters such as Rh2 for driving GOI expression specifically in Drosophila ocelli; and inducible promoters such as Mtn and DmHsp70 for achieving expression in the presence of Cu+ and in response to heat stress, respectively. 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.

Our Drosophila Cas9 expression pUASTattB vectors are designed to achieve efficient φC31 integrase-mediated genomic insertion of a Cas9 gene. Our vectors are optimized for high copy number replication in E. coli and high-efficiency transgenesis of Drosophila lines. The user-defined promoter version of this vector allows users to select a ubiquitous, tissue-specific, or inducible promoter for driving Cas9 expression.

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 efficiency: Achieving germ-line transgenesis using φC31 integrase vectors is more efficient than P-element based systems such as pUAST.

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.

Promoter: A DNA sequence upstream of a gene to which proteins bind to initiate transcription of that gene.

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.