Role of the Leader Sequence during Thermal Repression of Translation in Maize, Tobacco, and Carrot Protoplasts

Abstract

The 5'-untranslated leader of maize (Zea mays) heat-shock protein (hsp) 70 mRNA is required for translational competence during heat shock in protoplasts. When the #-glucuronidase gene was used as a reporter mRNA, expression at elevated temperatures increased more than 10-fold when the hsp70 leader constituted the 5'-untranslated region. The hsp7o leader did not affect the physical half-life of the mRNA and, therefore, does not function at the level of transcript stability. The maize hsp7O leader was required to escape thermal repression in both maize and tobacco (Nicotiana tabacum) but was less effective in carrot. In addition, mRNAs containing the tobacco mosaic virus untranslated leader (Q) were also efficiently translated during heat shock, data suggesting that the presence of the Q sequence enables the transcript to escape the translational repression that occurs during thermal stress.When organisms as diverse as mammals, bacteria, and plants are exposed to thermal stress, they respond by synthesizing a set of proteins, the hsps4 (12, 18). This phenomenon is under complex regulatory controls exerted at both the transcriptional and translational levels. As a consequence of thermal stress, the transcription of heat-shock genes, which were previously either silent or expressed at low levels, is increased both dramatically and rapidly, whereas other genes are transcriptionally repressed. Moreover, heat-shock messages are preferentially translated over the preexisting cellular mRNAs at elevated temperatures (12).The transcriptional regulation of heat-shock genes has been highly conserved throughout eukaryotes, including plants (9,16,18,19).

Untranslated Regions from C4 Amaranth AhRbcS1 mRNAs Confer Translational Enhancement and Preferential Bundle Sheath Cell Expression in Transgenic C4Flaveria bidentis

“…Translational enhancement is primarily known from some plant and animal viral RNAs (Gallie, 1993(Gallie, , 2002De La Luna et al, 1995;Chizhikov and Patton, 2000;Salvatore et al, 2002). There are also several examples of plant cellular transcripts showing enhanced translation capabilities (Pitto et al, 1992;Gallie and Young, 1994;Bate et al, 1996;Ling et al, 2000). Translational enhancement of cellular and viral mRNAs is often mediated through sequences located in their 5#-or 3#-UTRs and in some cases involves interactions with specific cellular proteins.…”

Induction of HSP70 and polyubiquitin expression associated with plant virus replication

“…23), and, following heat treatment, polyu- biquitin and HSP expressions are both induced. Heat-induced stress also results in the degradation of host mRNAs against which HSP (and presumably polyubiquitin) RNAs are protected (26). Hence, many of the features of the response to PSbMV replication mimic those resulting from heat stress.…”

Heat Shock Disrupts Cap and Poly(A) Tail Function during Translation and Increases mRNA Stability of Introduced Reporter mRNA

“…We know that regulatory elements defined for endogenous mRNAs [e.g. a cap and poly (A) tail] also act to regulate expression of introduced mRNA, that the presence of a ksp70 5'-leader sequence in an introduced reporter mRNA confers translational competence during heat shock (Pitto et al, 1992), and that introduced luc mRNA is released from polysomes following a heat shock (D.R. Gallie, unpublished observations), as has been observed for endogenous mRNAs (Key et al, 1981), and co-fractionates with the organellar fraction, consistent with the observation that nonheat-shock mRNAs move into nuclear-associated HSGs following heat shock (Nover et al, 1983(Nover et al, , 1989.…”