Over at his eponymous blog, Mike the Mad Biologist has a post suggesting that perhaps signal peptides are the fitness cost of antibiotic resistance in bacteria. Signal peptides, as he points out, have long been considered unimportant -- "for those of you who remember cassette tapes, signal peptides were thought to be like the white part at the beginning of the tape," he writes -- but his own work indicates that these peptides do indeed have a significant effect on protein function.
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