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Zusammenfassung:
Extrachromosomal genetic elements generally exhibit increased AT-contents relative to their hosts’ DNA. The AT-bias of endosymbiotic genomes is commonly explained by neutral evolutionary processes. Here we show experimentally that an increased AT-content of host-dependent elements can be selectively favoured on the host level. Manipulating the nucleotide composition of bacterial cells by introducing A+T- or G+C-rich plasmids, we demonstrate that cells containing GC-rich plasmids are less fit than cells containing AT-rich plasmids. Moreover, the cost of GC-rich elements could be compensated by providing G+C-, but not A+T-precursors, thus linking the observed fitness effects to the cytoplasmic availability of nucleotides. Our work identifies selection as a strong evolutionary force that drives the genomes of intracellular genetic elements toward higher A+T contents.
