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The mitogen-activated protein kinase (MAPK) cascade in Xenopus oocytes exhibits an all-or-none, ultrasensitive response, which is believed to result from a positive feed-back loop. Here we describe a context where 1,10-phenanthroline slowly, but strongly, activates MAPK while it impairs protein synthesis in a zincdependent manner, abolishing any feed-back loop. The induced-MAPK response was found to be strongly ultrasensitive. We argue that underlying this behaviour is a regulation motif akin to a feed-forward loop acting in vivo.
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