msDNA: the story of a unique satellite DNA molecule in bacteria

msDNA, or multi-copy single-stranded DNA, is a small satellite DNA observed in DNA extracts from a bacterial cell. A genetic element, usually found on the chromosome, called a retron, is responsible for producing msDNA. This small single-stranded DNA has a unique structure and is synthesized by reverse transcription from an RNA template. Retrons and msDNA are found in a large variety of different bacterial species. The msDNA molecule has also been developed into an important tool for genetic engineering and genome editing. But the biological function of this unique DNA molecule remained a mystery for decades. Recent discoveries show that a retron element can protect bacteria from infection with a virus (phage) using an abortive infection strategy. And the msDNA molecule is required for this anti-phage property. A historical perspective on this unusual DNA molecule, recent breakthroughs into its functions, and a discussion of the current and possible future directions of study are presented herein.