Diauxic shift-dependent relocalization of decapping activators Dhh1 and Pat1 to polysomal complexes

Drummond, Sheona P.; Hildyard, John; Firczuk, Helena; Reamtong, Onrapak; Li, Ning; Kannambath, Shichina; Claydon, Amy J.; Beynon, Robert J.; Eyers, Claire E.; McCarthy, John E. G.

The imaging and polysomal gradient data indicate collectively that Dhh1 and Pat1 are granted only restricted access to actively translating mRNP during exponential growth. This fits with the concept of co-translational mRNA decay, as originally observed in bacteria and more recently reported in yeast (14) , since this suggests that these proteins can, in response to the diauxic growth shift, gain access to mRNAs that are being translated and then promote decapping. This would mean that Dhh1 and Pat1 would only be capable of fully promoting decapping (and inhibiting translation, possibly involving an additional mechanism) on polyribosomal mRNPs under conditions where the cell needs to reduce its rate of protein synthesis. However, the data reported here do not suggest that attenuation of translation per se is the determinant of the re-localization of Dhh1 and Pat1 or of the increased access of these proteins to polysomal mRNP complexes in the cell. Moreover, P body formation (as detected by incorporation of Dhh1-GFP) is not promoted by inhibition of translation via suppression of translation factor activities using tet07-dependent regulation. It is of interest that Teixeira and colleagues have reported (32) that inhibition of translation initiation by switching a temperature sensitive prt1-63 strain from 23 to 37 C causes P-bodies to increase in size. Thus, more extreme conditions than those applied in this work, i.e. complete inhibition of translation induced by temperature stress, can have an effect on (the rate of) P-body formation. Overall, it seems that the remodelling of translationally active mRNPs we have described here is likely to be triggered by a diauxic growth-shift-induced molecular pathway that functions independently of the rate at which the translation machinery is functioning. The enhanced access of Dhh1 and Pat1 to actively translating polysomes is consistent with an attenuated requirement for post-transcriptional expression of many genes when cells encounter poorer growth conditions.