Quantitative variations in the level of MAPK activity control patterning of the embryonic termini in Drosophila

We have examined the role in patterning of quantitative variations of MAPK activity in signaling from the Drosophila Torso (Tor) receptor tyrosine kin ase (RTK). Activation of Tor at the embryonic termini leads to differential expression of the genes tailless and huckebein. We demonstrate, using a se ries of mutations in the signal transducers Corkscrew/SHP-2 and D-Raf, that quantitative variations in the magnitude of MAPK activity trigger both qua litatively and quantitatively distinct transcriptional responses. We also d emonstrate that two chimeric receptors, Tor(extracellular)-Egfr(cytoplasmic ) and Tor(extracellular)-Sev(cytoplasmic) cannot fully functionally replace the wild-type Tor receptor, revealing that the precise activation of MAPK involves not only the number of activated RTK molecules but also the magnit ude of the signal generated by the RTK cytoplasmic domain. Altogether, our results illustrate how a gradient of MAPK activity controls differential ge ne expression and, thus, the establishment of various cell fates. We discus s the roles of quantitative mechanisms in defining RTK specificity. (C) 199 9 Academic Press.