Nucleolar proteins regulate stage-specific gene expression and ribosomal RNA maturation in Trypanosoma brucei

Different life-cycle stages of Trypanosoma brucei are characterized by stage-specific glycoprotein coats. GPEET procyclin, the major surface protein of early procyclic (insect midgut) forms, is transcribed in the nucleolus by RNA polymerase I as part of a polycistronic precursor that is processed to monocistronic mRNAs. In culture, when differentiation to late procyclic forms is triggered by removal of glycerol, the precursor is still transcribed, but accumulation of GPEET mRNA is prevented by a glycerol-responsive element in the 3' UTR. A genome-wide RNAi screen for persistent expression of GPEET in glycerol-free medium identified a novel protein, NRG1 (Nucleolar Regulator of GPEET 1), as a negative regulator. NRG1 associates with GPEET mRNA and with several nucleolar proteins. These include two PUF proteins, TbPUF7 and TbPUF10, and BOP1, a protein required for rRNA processing in other organisms. RNAi against each of these components prolonged or even increased GPEET expression in the absence of glycerol as well as causing a significant reduction in 5.8S rRNA and its immediate precursor. These results indicate that components of a complex used for rRNA maturation can have an additional role in regulating mRNAs that originate in the nucleolus.

Michel Butor à la frontière

Dans l’œuvre de Michel Butor, la frontière peut être considérée comme un équivalent de la césure, comme le pli de la double page constituait un équivalent de la césure dans Un coup de dés chez Mallarmé. Elle se retrouve dans de très nombreuses œuvres, des romans au Génie du lieu, et dans de nombreuses œuvres poétiques, dont À la frontière et Frontières. Chez Butor, la frontière a un rôle d’articulateur ; mais cette articulation ne va pas de soi, et il faut passer par des «méditations» sur la frontière, qui permettent d’établir une typologie des diverses fonctions de la frontière.]