Enzyme necessary for DNA synthesis can also erase DNA

The amount of DNA in the chromosomes of bacteria can change rapidly, either by increasing, so-called gene amplification, or by decreasing, so-called gene deletion. These processes are evolutionarily extremely important, and the discovery of a new mechanism that is involved when DNA disappears is of great importance to our understanding what influences the stability of chromosomes and why the amount of DNA can decline in certain types of bacteria.

“How rapidly and by what mechanisms DNA can disappear from the chromosome is a central genetic and evolutionary question,” says Professor Dan I Andersson, who is the lead author of the study.

Previously these types of drastic gene reductions, deletions, have mostly been studied in artificial model systems with two long identical and neighboring DNA sequences. Normal spontaneous deletions, on the other hand, are produced in another way, and what’s more, the DNA sequences are often a remote from each other.

In the current study doctoral candidate Sanna Koskiniemi has carried out comprehensive genetic analyses of Salmonella mutants and shows that a special type of DNA-synthesizing enzymes are necessary if spontaneous deletions are to be formed in the bacteria. This new function has never before been described in these enzymes. By genetically inactivating or overproducing these enzymes, these researchers were able to show that the deletion rapidity decreased or increased by up to 30 times.

“These findings can explain how and why the DNA content of different organisms varies and what genetic mechanisms govern this,” says Professor Dan Andersson, who maintains that bacteria that live either as parasites inside cells or in symbiosis with other organisms are of special interest.

“They are often small chromosomes because DNA has disappeared in the course of evolution. With these new findings we can better understand and predict how DNA is eliminated from chromosomes.”