Huge viruses with CRISPR-Cas and translational components have been described by researchers at the University of California, Berkeley – providing a possible new tool for genome editing of bacteria.

The study collected metagenomic data from a wide range of sources and discovered 35 giant versions of bacteriophages with genes encoding undescribed CRISPR-Cas systems and ribosomal parts – not found in typical viruses. The environmental sources came from across the globe covering soil, water, and faecal samples of various origins.

Bacteriophages are a type of virus that infect bacteria and are typically classified as ‘non-living’ due to the innate lack of replication and the ability to translate their own genetic material without a host. These phages can have effects on the surrounding ecosystem by facilitating gene transfer between bacteria and spreading antibiotic resistance.

Typically, bacteriophages are small with an average genome of 52,000 bp, whereas these 35 newly described phages genomes are almost 4 times larger. The largest discovered has a genome of 735,000 base pairs.

These new huge phages were then classified into 10 new clades depending on their size and components, among them Whopperphage, Biggiephage, and Enormephage, implying size may be everything in the world of phage clades.

Bacteroidetes seem to be the target of these huge phages, a type of bacteria that is widely distributed among the environment and mostly benign. They are especially abundant in our guts where they are viewed as the most stable part of our microbiome and a reduction in numbers has been associated with obesity.

The CRISPR-Cas systems discovered in the phages differ from the usual CRISPR-Cas9 by utilising different Cas proteins or simply the proteins of their host. This system has the capacity to silence host transcription factors and some genes. Thus, offering a new avenue for altering the genomes of bacteria by either modulating transcription or potentially triggering a kill switch.

As the main target of these huge phages may be our microbiome, harnessing the potential genome editing of these new clades could improve human health.

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