Starved Saccharomyces cerevisiae Cells Have the Capacity to Support Internal Initiation of Translation

Abstract

Internal initiation of translation, whereby ribosomes are directed to internal AUG codon independently of the 5 end of the mRNA, has been observed rarely in higher eucaryotes and has not been demonstrated in living yeast. We report here that starved yeast cells are capable of initiating translation of a dicistronic message internally. The studied element that functions as an internal ribosome entry site (IRES) is hardly functional or not functional at all in logarithmically growing cells. Moreover, during the logarithmic growth phase, this element seems to inhibit translation reinitiation when placed as an intercistronic spacer or to inhibit translation when placed in the 5-untranslated region of a monocistronic message. Inhibition of translation is likely due to the putative strong secondary structure of the IRES that interferes with the cap-dependent scanning process. When cells exit the logarithmic growth phase, or when artificially starved for carbon source, translation of the IRES-containing messages is substantially induced. Our findings imply that the capacity to translate internally is a characteristic of starved rather than vegetatively growing yeast cells.The ribosome scanning model has been originally proposed by Kozak (1) to explain how the translation process is initiated. Numerous studies have corroborated the model whereby the initiation complex is assembled near or at the 5Ј end of the mRNA, facilitated by the interaction of the cap structure with the eucaryotic initiation factor 4E, and starts scanning the mRNA until the first AUG is encountered (for recent review see Ref. 2). An alternative mode of selecting an initiation codon, whereby ribosomes are directed to an internal AUG by an internal ribosome entry sequence (IRES), 1 has also been demonstrated. Well documented cases of internal initiation events are those of the uncapped picornaviral mRNAs (3). IRESes have also been found in the 5Ј-untranslated region (5Ј-UTR) of several cellular mRNAs (4 -13). During evolution, IRESes have been utilized as targets for translation regulation during normal differentiation and development. For example, an IRES was shown to play a role in the translation of platelet-derived growth factor 2 mRNA that increases after megacaryocitic cells undergo terminal differentiation (10). IRESes have also been found to mediate the differential translation of Antenapedia and Ultrabithorax during Drosophila melanogaster development (6,12). Surprisingly, no IRES has been shown to function in yeasts, despite the observations that the yeast cell-free system is capable of recognizing plant viral IRESes (14) as well as natural yeast leader sequences (15). Attempts to promote internal initiation in living yeast cells by using IRESes of poliovirus (16) and encephalomyocarditis virus (17) have thus far failed. These studies were done with optimally growing cells. In their natural environment, however, yeast occasionally encounter starvation and enter into a distinct quiescent state called stationary phase (SP) (reviewed in Refs. ...

Isolation and identification of short nucleotide sequences that affect translation initiation in Saccharomyces cerevisiae

“…Other studies have identified IRESes within the yeast TFIID and HAP4 mRNAs, which were shown to function in yeast cell-free lysates (21), and in the YAP1 and p150 mRNAs, which we showed can function in vegetatively growing cells (22). The ability of yeast cells to initiate translation internally is also supported by the observations that RNA sequences from various organisms and sources, including the cricket paralysis virus, can function as IRESes in yeast (23)(24)(25).…”

“Sleeping Beauty”: Quiescence inSaccharomyces cerevisiae

“…It is clear that many of the cells in a colony growing on the surface of solid rich media are also in quiescence (108). However, because of the immobility of yeast cells on solid surfaces, distinct microenvironments develop within a colony, allowing multiple subpopulations of cells to coexist.…”

Hepatitis C Virus Internal Ribosome Entry Site-Dependent Translation in Saccharomyces cerevisiae Is Independent of Polypyrimidine Tract-Binding Protein, Poly(rC)-Binding Protein 2, and La Protein

“…Initiation dependent upon the HCV and poliovirus IRESes has been observed in cell extracts from yeast (4,26,27). IRESdependent initiation occurs in yeast that have undergone carbon starvation (51) or when 5Ј-end-dependent initiation is compromised (38,65). At least one gene of S. cerevisiae, URE2, harbors an IRES (38).…”