Such considerations are leading molecular biologists to pay much more attention to weak and relatively unselective interactions in the genome. Many of these involve proteins that, instead of having a fixed and precise structure, are loose and floppy \u2014 \u201cintrinsically disordered,\u201d as biochemists put it. If proteins only worked through rigid structural domains, Young explained, it would constrain not only how well regulatory systems could evolve but also the kinds of dynamic regulation seen in life. \u201cYou won\u2019t find a living organism \u2014 or even a virus \u2014 functioning with only stable structural elements like those in a Swiss watch,\u201d Young said.<\/p>\n

Perhaps evolution just stumbled on STRs as a component of such a complex but ultimately more effective solution to gene regulation in eukaryotes. STRs themselves may arise in several ways \u2014 for example, through errors in DNA replication or the activity of DNA segments called transposable elements that make copies of themselves throughout the genome.<\/p>\n

\u201cIt just so happened that the resulting emergent weak interactions between proteins and the repetitive sequences was something that could \u2026 provide selective advantage to the cells where it occurred,\u201d Kuhlman said. His guess is that this fuzziness was probably forced upon eukaryotes, but that \u201cthey were subsequently able to exploit [it] for their own benefit.\u201d Bacteria and other prokaryotes can rely on well-defined \u201cdigital\u201d regulatory logic because their cells tend to exist in only a few simple, distinct states, such as moving around and replicating.<\/p>\n

But the different cell states for metazoans are \u201cmuch more complex and sometimes close to a continuum,\u201d Suter said, so they are better served by fuzzier \u201canalog\u201d regulation.<\/p>\n

\u201cThe gene regulatory systems in bacteria and eukaryotes do seem to have diverged quite substantially,\u201d Tjian agreed. While Monod is said to have once remarked that \u201cwhat is true for E. coli<\/em> is true for the elephant,\u201d it seems that isn\u2019t always so.<\/p>\n<\/div>\n

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