@greggillotti Sure. Not sure at what level to summarize, so forgive me if oversimplified or overcomplicated. Happy to clarify if so. Ok:
@greggillotti Background info: Plasmids are circular DNA strands that aren't part of an organism's chromosomal genome. They are indepent genetic elements that get replicated within cells, usually bacteria, on their own and get passed to the next generation after cell division, too.
@greggillotti In addition to being copied and passed on from parent cell to child cell, these plasmids are a key component of bacterial evolution (e.g. antibiotic resistance) that occurs via horizontal transfer, in this case typically conjugation: different bacteria exchange material via...
@greggillotti ...direct contact. This paper examined the mechanisms by which bacteria compensate for the cost of harboring plasmids.
@greggillotti Rather than general factors like DNA strand length (which varies widely among plasmids) that might impact ability for bacteria to harbor plasmid DNA, the authors identify much more specific control of plasmid compensatory evolution.
@greggillotti They use one species of bacteria and several plasmids to measure fitness of the bacterial cells when the plasmids are introduced. They put the plasmids in normal version of the bacteria, a genetically modified version knocking out the genes they hypothesized were involved, and ..
@greggillotti ... compared their growth to bacterial cells of wild type and mutated strains but without the plasmids. They found that there was fitness cost to harboring new plasmids, but those costs were mitigated in the mutated cell lines, most notably mutation to PFLU4242.
@greggillotti Moreover, it's not just that mutations to gacS or PFLU4242 were generally beneficial to cell fitness. Cell lines with those mutations but without plasmids introduced did not show growth levels different from wild type. Rather, the interaction of those genes with plasmid DNA...
@greggillotti ... confers the change in fitness and growth for the bacteria, true for each of the different plasmids they used in this case.
@greggillotti They then went on to further examine how these interactions might work and propose this illustration of those interactions.
@greggillotti TLDR: PFLU4242 is regulated by gacS in this species, and mutation to chromosomal PFLU4242 can pretty much mitigate the cost of taking on plasmid freeloaders. This ease of compensation to having plasmid is then a driver of bacterial evolution, facilitating horizontal gene transfer

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