Translational repression of mRNA for eucaryotic elongation factors in Friend erythroleukemia cells

Lymphocyte p56L32 is a RNA/DNA-binding protein which interacts with conserved elements of the murine L32 ribosomal protein mRNA

In previous studies of the ribosomal protein L32 mRNA, we demonstrated that a conserved polypyrimidine tract found in the 5'-untranslated region (5'-UTR) was required for translational regulation in vivo and that a 56-kDa protein (p56L32) from T-lymphocytes specifically interacts with this sequence [Kaspar, R. L., Kakegawa, T., Cranston, H., Morris, D. R. & White, M. W. (1992) J. Biol. Chem. 267, 508-514]. Here we show that p56L32 binding to the L32 5'-UTR is complex and requires other 5'-UTR RNA sequences in conjunction with the polypyrimidine tract. Deletion and site-directed mutagenesis studies revealed that binding of p56L32 to the L32 5'-UTR requires a second RNA element, GGUGGCUGCC, 15 nucleotides downstream from the polypyrimidine tract. In contrast, L32 RNA transcripts altered in this downstream element were good substrates for binding of the polypyrimidine binding proteins from HeLa nuclear extracts, indicating that these proteins have RNA-binding specificities distinct from p56L32. Competition analysis demonstrated that p56L32 will bind to DNA as well as RNA with identical sequence specificity and similar affinity. Single or double-stranded DNAs composed of the L32 5'-UTR sequences were found to specifically compete with L32 RNA transcripts for p56L32 binding. The L32 5'-UTR downstream element, GGUGGCUGCC, which is required for p56L32 binding, has previously been implicated as a transcriptional element of the L32 gene. The ability of p56L32 to bind this sequence as DNA or RNA suggests p56L32 may have a dual role in the regulation of ribosomal protein mRNA accumulation and translation.

Translational repression of EF-1 alpha mRNA in vitro

In this report we show that when 10,000 x g supernatant extracts of growth arrested murine erythroleukemia (MEL) cells are incubated there is a rapid conversion of essentially all mRNAs to non-translating messenger ribonucleoprotein (RNP) particles. Most of these RNPs are readily translated in an initiation-dependent manner when added to a nuclease-treated rabbit reticulocyte lysate. A notable exception is the RNP containing eucaryotic elongation factor 1 alpha (EF-1 alpha) mRNA. The mRNA for poly(A)-binding protein behaved similarly to EF-1 alpha. Previous work has demonstrated that the translation of both these mRNAs are repressed in vivo when the growth of a number of different mammalian cells is arrested [Slobin L. I. and Jordan, P. (1984) Eur J. Biochem. 145, 1984; Thomas, G. and Thomas, G. (1986) J. Cell Biol. 103, 1986]. Translational activity of EF-1 alpha mRNA could be restored by treating RNP particles with 0.5 M KCl, provided that the RNPs were separated from salt wash by chromatography on oligo(dT)-cellulose. Addition of the salt wash to total MEL cell mRNA significantly and selectively inhibited EF-1 alpha mRNA translation, suggesting that a component of the salt wash acts as a trans-acting translational repressor of EF-1 alpha mRNA.

Distribution and activity of alternatively spliced Alzheimer amyloid peptide precursor and scrapie PrP mRNAs on rat brain polysomes

Mammalian brains contain low levels of the Alzheimer amyloid precursor variants (AAPPs) and the normal form of the scrapie agent protease-resistant protein (PrPc); however, their mRNAs are readily detectable. To understand these discrepancies we have investigated some aspects of the translational regulation of these mRNAs. An accurate blot-hybridization procedure was developed to measure absolute amounts of mRNA. Rat brain contains the following mRNA levels (ng/g tissue) AAPP(695), 170; AAPP(751/770), 63; PrPc, 144; actin, 615; glyceraldehyde-3-phosphate dehydrogenase (G3PDH), 359; ferritin, 148. The method was also used to determine the distribution of mRNAs between translationally active polysomes and translationally inactive ribonucleoprotein protein particles (mRNPs). More than 90% of G3PDH and actin mRNAs were associated with polysomal RNA; whereas, ferritin light chain mRNA was predominantly (90%) in mRNP RNA. The degree of cross-contamination of mRNPs with polysomes was less than 10%. Probes specific for the scrapie PrP protein and the AAPP(695) splice junction revealed that 70% of these mRNAs were associated with polysomes. One-half of AAPP(751/770) mRNAs (which comprise 20-30% of all AAPP mRNA in brain) were found in polysomes. We conclude therefore that both scrapie and AAPP mRNAs are subject to translational regulation in rat brain. Evidence from in vitro translational experiments confirm the message distribution determined by blot hybridization and corroborate the hypothesis that AAPP is subject to partial post-transcriptional regulation. Nevertheless, the low tissue levels of AAPP and PrPc must result primarily from their relatively rapid turnover.

Increase in abundance of a transcript hybridizing to elongation factor I alpha during cellular senescence and quiescence

We have isolated a senescence-specific clone (pSEN) from a cDNA library constructed from late passage WI-38 human diploid fibroblast that accounts for approximately 1% of the recombinants. Nucleotide sequence analysis of the partial cDNA clone has led to the identification of pSEN as elongation factor I alpha. Northern analysis of poly(A)+ RNA from various intermediate population doubling levels shows that a 2.2 kb transcript hybridizes to pSEN but is expressed prior to PDL-40 at very low levels. This transcript begins to accumulate at PDL-40 and is induced approximately 50-fold just prior to senescence. Furthermore, this transcript was shown to be specific to Go of the cell cycle whereas a second, lower molecular weight transcript (1.6 kb) was observed during S phase (Giordano and Foster, unpublished data). The 2.2 kb transcript is also detected in neonatal foreskin cells but very little increase in abundance is observed between early and late passage cells. Sucrose gradient fractionation of RNA from late passage WI-38 cells suggests that the lower molecular weight transcript is associated with the polysome fraction while the 2.2 kb transcript sediments with the nonpolysomal fraction. Thus, the possibility exists that the 1.6 kb transcript is derived from the 2.2 kb transcript.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

An in vitro system derived from Friend erythroleukemia cells to study messenger RNA stability

A system consisting of 40-80S messenger ribonucleoprotein particles (mRNP) from stationary Friend erythroleukemia (FEL) cells was used to investigate the stability of mRNA in vitro. The majority of mRNP mRNAs were found to be stable when incubated for periods of up to ninety minutes at 37 degrees. Nonetheless, many mRNAs are greatly reduced in abundance, including ones for eucaryotic elongation factor Tu (eEF-Tu) and the 73-78 kDa polypeptide commonly found in association with the poly(A) tails of mRNA. A divalent cation dependent ribonuclease (probably an endoribonuclease) could be washed off mRNP by treatment of the particles with 0.5M NaCl. The mRNAs contained in the resultant salt washed mRNPs, including eEF-Tu, were stable when incubated in vitro.

Regulation of the utilization of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells

When Friend erythroleukemia cells were allowed to grow to stationary phase (2 X 10(6) to 3 X 10(6) cells per ml), approximately 60% of the mRNA for eucaryotic elongation factor Tu (eEF-Tu) sedimented at less than or equal to 80S, and most of the remaining factor mRNA was associated with small polysomes. Under the same growth conditions, greater than 90% of the mRNA for eucaryotic initiation factor 4A remained associated with polysomes. The association of eEF-Tu mRNA with polysomes changed dramatically when stationary-phase cells were treated with fresh medium. After 1 h in fresh medium, approximately 90% of eEF-Tu mRNA in Friend cells was found in heavy polysomes. Associated with the shift of eEF-Tu mRNA into heavy polysomes, we found at least a 2.6-fold increase in the synthesis of eEF-Tu in vivo as well as a remarkable 40% decrease in the total amount of eEF-Tu mRNA per cell. Our data raise the possibility that eEF-Tu mRNA that has accumulated in ribonucleoprotein particles in stationary-phase cells is degraded rather than reutilized for eEF-Tu synthesis.

Regulation of the utilization of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells

When Friend erythroleukemia cells were allowed to grow to stationary phase (2 X 10(6) to 3 X 10(6) cells per ml), approximately 60% of the mRNA for eucaryotic elongation factor Tu (eEF-Tu) sedimented at less than or equal to 80S, and most of the remaining factor mRNA was associated with small polysomes. Under the same growth conditions, greater than 90% of the mRNA for eucaryotic initiation factor 4A remained associated with polysomes. The association of eEF-Tu mRNA with polysomes changed dramatically when stationary-phase cells were treated with fresh medium. After 1 h in fresh medium, approximately 90% of eEF-Tu mRNA in Friend cells was found in heavy polysomes. Associated with the shift of eEF-Tu mRNA into heavy polysomes, we found at least a 2.6-fold increase in the synthesis of eEF-Tu in vivo as well as a remarkable 40% decrease in the total amount of eEF-Tu mRNA per cell. Our data raise the possibility that eEF-Tu mRNA that has accumulated in ribonucleoprotein particles in stationary-phase cells is degraded rather than reutilized for eEF-Tu synthesis.

Genes coding for the elongation factor EF-1 alpha in Artemia

The elongation factor EF-1 alpha is one of the most abundant proteins in eukaryotic cells, where it catalyzes the binding of aminoacyl-tRNA to ribosomes. The genes coding for this protein in the brine shrimp Artemia were analyzed by gene cloning, electron microscopy and chromosomal blot hybridization. There are only a few (about four) copies of one type of gene per haploid genome. These genes contain five exons divided over 10(4) base pairs. Local rearrangements give rise to a number of gene variants. Cross-hybridizations of Artemia cDNA probes with yeast and Drosophila DNA revealed two different yeast EF-1 alpha genes and one or two different Drosophila genes, respectively. Nucleotide sequencing revealed signals for synthesis and processing of EF-1 alpha transcripts as well as the exact location of exons. One interruption in the coding sequence corresponds closely to a splice junction in the gene coding for the homologous chloroplast protein EF-Tu from Euglena gracilis, presumably of prokaryotic origin. The first exon in the chloroplast gene codes for the region of EF-Tu that is homologous to regions of the elongation factor EF-G and of the initiation factor IF2, respectively.