transposon directed insertion-site sequencing is a useful genetic tool that can be used to identify key genes in a host organisms genome ## The system TraDIS standardly uses the Tn5-mini transposon, it doesnt contain a site of origin so it can only be inserted into the chromosome once the transposon has 3 important components - a selection marker, generally an antibiotic resistant marker - inverted repeats that flank the transposon - detectable probe/DNA marker that can then be used to map the genome later there wil then also be the transposase. this is transfected into the cell alongside the transposon. it will recognise the inverted repeats, induced double stranded breaks and then randomly (as much as possible there is sometimes a smalll degree of preference for regions rich in certain base pairs) inserts the fragment into the genome. the transposon is only wanted to be inserted into the genome once so the phenotype (especially if used in further screenings) can be mapped to hopefully a single gene disruption event. this also means that alternative systems using larger plasmids (this could mean that the plasmid has the transposase and the transposon in one system) will use a suicide vector. confusingly or at least to me this has nothing about inducing cell death **only that the vector can not be replicated inside the host cell** so the site of origin of the plasmid may be one that isnt recognised and thus the one transposon present in the plasmid is the only thing (outside random mutation and weird other factors yk) that is going to induce gene disruption. this system is then used to generate a library of mutants, hopefully with mutatitons in most if not all of the genome. this library is then sequenced. for TraDIS to work there must be a reference genome. if you are looking for genes of **essential function what you would see when these genes are knocked out is that the number of reads in the library at that position are low**, backwards to what you might expect this to work. because the cells do not survive well or die because the genes are essential there is not as many of them in the pool of cells you are looking for. these gaps can then be compared to the reference library to find the corrosponding genes. This can be done in various conditions to find genes vital in certain scenarios. you could also do this in knock-out cell lines to uncover genes that are vital in redundancy. ## Limitations Genes can falsely be identified as essential if they have a low read count even if they are non-essential. it may be that the site is blocked by proteins bound to the chromosome or that the transposon interupts down stream expression of genes (polar inserts) or even just that the gene impacts growth rate. <u>**References**<u/> - [Warner, I., Kok, W., Martinelli, N., Yang, Z., Emily and Henderson, I.R. (2023). Microbial Primer: Transposon directed insertion site sequencing (TraDIS): A high throughput method for linking genotype to phenotype. _Microbiology_, 169(11). doi:https://doi.org/10.1099/mic.0.001385.](https://pmc.ncbi.nlm.nih.gov/articles/PMC10710833/)