Certain transposons have characteristic end sequences, as well as encoding a set of genes required for transposition. A search of a whole genome sequence will often turn up recognizable transposable elements. CRISPR/Cas systems are also recognizable by the CRISPR repeat array and adjacent Cas operon. A survey of bacterial chromosomes revealed that many organisms have a DNA region that is a variant CRISPR/Cas system called Cas7f and a Tn7-like transposon. Cas7f contains a short CRISPR array and a subset of Cas genes. It lacks the gene encoding the interference nuclease, meaning that it directs an effector complex to a site on the chromosome specified by the array, but cannot cut the DNA at that site. Cas7f is located within a Tn7-like transposon. The transposon is also a variant, in that it has its characteristic repeats at each end of the element, and the genes encoding transposase, but it lacks the gene encoding the protein that directs transposition to plasmids and bacteriophages. Researchers realized that they were observing a DNA sequence recognition module with no enzymatic activity (Cas7f) and a transposon that had lost its ability to identify a target DNA sequence. They proposed that this new genetic element represents an RNA-guided transposon.
Diagram how transposition might occur for this variant transposon.
Certain transposons have characteristic end sequences, as well as encoding a set of genes required for transposition. A search of a whole genome sequence will often turn up recognizable transposable elements. CRISPR/Cas systems are also recognizable by the CRISPR repeat array and adjacent Cas operon. A survey of bacterial chromosomes revealed that many organisms have a DNA region that is a variant CRISPR/Cas system called Cas7f and a Tn7-like transposon. Cas7f contains a short CRISPR array and a subset of Cas genes. It lacks the gene encoding the interference nuclease, meaning that it directs an effector complex to a site on the chromosome specified by the array, but cannot cut the DNA at that site. Cas7f is located within a Tn7-like transposon. The transposon is also a variant, in that it has its characteristic repeats at each end of the element, and the genes encoding transposase, but it lacks the gene encoding the protein that directs transposition to plasmids and bacteriophages. Researchers realized that they were observing a DNA sequence recognition module with no enzymatic activity (Cas7f) and a transposon that had lost its ability to identify a target DNA sequence. They proposed that this new genetic element represents an RNA-guided transposon.
Diagram how transposition might occur for this variant transposon.
