No abstract

Xlsirts are a family of interspersed repeat RNAs from Xenopus laevis that contain from 3 to 13 repeat units (each 79 to 81 nucleotides long) flanked by unique sequences. They are homologous to the mammalian Xist gene that is involved in X chromosome inactivation. Xlsirt RNA appears first in the mitochondrial cloud (Balbiani body) in stage 2 oocytes and is then translocated as island-like structures to the vegetal cortex at early stage 3 coincident with the localization of the germ plasm. Exogenous Xlsirt RNA injected into oocytes translocates to the location of the endogenous RNA at that particular stage. The Xlsirt RNA repeat sequences are required for translocation and can cause the translocation of heterologous unique RNAs to the vegetal cortex.

No abstract

Messenger RNA (mRNA) and messenger like RNA (mlRNA) were investigated in the cytoplasm of HeLa cells while ribosomal RNA synthesis was arrested. Under these conditions, functional mRNA associated in polyribosomes and cytoplasmic free mlRNA are formed and can be labelled selectively to steady state.All cytoplasmic non-ribosomal RNA sedimenting a t more than 6-7 S exists in the form of ribonucleoprotein complexes which pre-exist in the cell, and are stable upon cell lysis, sedimentation and (after fixation) CsCl density gradient analysis. The functional, true mRNA is contained in a complex of mRNA and protein which bands in association with ribosomes (e = 1.52 to 1.60 g/cm3) in CsCl density gradients or, released by EDTA, a t its own intrinsic density of 1.40-1.88 g/cm3. The cytoplasmic free mlRNA bands as a particle of mlRNA-containing ribonucleoprotein at an identical low density. The molecular weight spectrum of mRNA is identical t o that of mlRNA and the sedimentation pattern of the mRNA-protein complex released from polyribosomes is similar to that of the free mlRNA -protein complex.The physico-chemical separation of mRNA -and mlRNA -protein complexes allowed us to follow their relative kinetics of synthesis and decay. Each type of ribonucleoprotein obeys a different, strictly time-dependent pattern. Label enters the pool of free mlRNA -protein complexes first and may, in a pulse-chase experiment, be partially chased into polyribosomes. At steady state (6 h) 40-60°/, of the labelled RNA remains in the form of free mlRNA-protein particles. These cannot be chased into polyribosomes, the kinetics of mRNA. and mlRNA-protein complexes decay following identical patterns. These findings are in agreement with a model according to which mlRNA from the nucleus first joins the pool of free ribonucleoprotein. Then, the activated mRNA-protein complexes attach to ribosomal subunits and form polyribosomes whereas inactivated mlRNA * protein complexes remain free in the cytoplasm.In order to further strengthen the evidence in favour of the real existence of mRNA-and mlRNA * protein complexes in the cells, the corresponding fractions from sedimentation or CsCl density gradients were observed in the electron microscope. By this method it was possible to see small cytoplasmic particles which have not before been identified. The rounded structures, with diameters ranging from 100 h to 200 8, seem to consist of the coiling of a 35 8 wide pearllike chain which may also be identsed in polyribosomes. The frequency of these particles is highest in the mlRNA. protein band (e = 1.40-1.48 g/cmS). Thus they may correspond to the mRNA -protein complex. However since they share some morphological features with other known biological structures the evidence is not conclusive.t We are desolated to announce to our friends and collegues that Nicole Granbouland died in an accident shortly before the completion of this manuscript. We dedicate this paper to her memory.Unusuul Abbreviations. mRNA, messenger RNA (this term is restricted to function...