Non‐AUG translation initiation of mRNA encoding acidic ribosomal P2A protein in Candida albicans

117

Although previous studies have already shown that both cytoplasmic and mitochondrial activities of glycyltRNA synthetase are provided by a single gene, GRS1, in the yeast Saccharomyces cerevisiae, the mechanism by which this occurs remains unclear. Evidence presented here indicates that this bifunctional property is actually a result of two distinct translational products alternatively generated from a single transcript of this gene. Except for an amino-terminal 23-amino acid extension, these two isoforms have the same polypeptide sequence and function exclusively in their respective compartments under normal conditions. Reporter gene assays further suggest that this leader peptide can function independently as a mitochondrial targeting signal and plays the major role in the subcellular localization of the isoforms. Additionally, whereas the short protein is translationally initiated from a traditional AUG triplet, the longer isoform is generated from an upstream inframe UUG codon. To our knowledge, GRS1 appears to be the first example in the yeast wherein a functional protein isoform is initiated from a naturally occurring non-AUG codon. The results suggest that non-AUG initiation might be a mechanism existing throughout all kingdoms.

Section: Discussionsupporting

confidence: 59%

Translation Initiation from a Naturally Occurring Non-AUG Codon in Saccharomyces cerevisiae

Chang

Wang

2004

117

“…Contrary to what happens in prokaryotes, initiation at GUG codons occurs rarely in higher eukaryotes (2). In fungi, a GUG start codon has been demonstrated for the C. albicans ribosomal protein P2A (8) and has also been proposed for other genes based on amino acid comparison of similar proteins, as is the case with ACV (␣-aminoadipyl-cysteinyl-valine) synthetase of Cephalosporium acremonium (32). In vitro experiments have also demonstrated that the Aspergillus nidulans areB gene may use a GUG codon as a minor initiation site (33).…”

Section: Discussionmentioning

confidence: 90%

“…So far, only three yeast genes have been demonstrated to initiate at native non-AUG codons: the Candida albicans gene coding for the acidic ribosomal protein P2A (CARP2A gene) (8) and the Saccharomyces cerevisiae genes GRS1 and ALA1, which code for glycyl-tRNA synthetase (GlyRS) and alanyl-tRNA synthetase (AlaRS), respectively (9,10). The GRS1 and ALA1 genes provide both the cytoplasmic and mitochondrial isoforms of the corresponding enzymes, the mitochondrial forms being initiated from non-AUG triplets upstream of the AUG initiators of the cytoplasmic counterparts.…”

mentioning

confidence: 99%

“…The GRS1 and ALA1 genes provide both the cytoplasmic and mitochondrial isoforms of the corresponding enzymes, the mitochondrial forms being initiated from non-AUG triplets upstream of the AUG initiators of the cytoplasmic counterparts. A single GUG codon seems to be used for initiation of CARP2A (8). None of these genes plays a regulatory role, whereas in higher eukaryotes a non-AUG codon acting as a sole start site is usually associated with regulatory mechanisms of gene expression for proteins such as proto-oncogenes (11) and transcriptional factors (12).…”

mentioning

confidence: 99%

See 1 more Smart Citation

Non-AUG Translation Initiation of a Fungal RING Finger Repressor Involved in Photocarotenogenesis

Murcia

Lorca-Pascual

Garre

et al. 2007

The RING finger protein CrgA acts as a negative regulator of light-induced carotene biosynthesis in the fungus Mucor circinelloides. Sequence analysis of the crgA coding region upstream of the first AUG codon revealed the existence of an additional non-canonical RING finger domain at the most N-terminal end of the protein. The newly identified RING finger domain is required for CrgA to regulate photocarotenogenesis, as deduced from site-directed mutagenesis experiments. The role of both RING finger domains in the stability of CrgA has been investigated in a yeast system. Wild type CrgA, but not the RING finger deleted forms, is highly unstable and is stabilized by inhibition of the proteasome function, which suggests that native CrgA is degraded by the proteasome and that active RING finger domains are required for proteasome-mediated CrgA degradation. To identify the translation start of CrgA, a mutational analysis of putative initiation codons in the 5 region of the crgA gene was accomplished. We demonstrated that a GUG codon located upstream of the first AUG is the sole initiator of CrgA translation. To our knowledge, this is the first report of a naturally occurring non-AUG start codon for a RING finger regulatory protein. A combination of suboptimal translation initiation and proteasome degradation may help to maintain the low cellular levels of CrgA observed in wild type cells, which is probably required for accurate regulation of photocarotenogenesis.

“…Perhaps for that reason, yeast cannot efficiently use non-AUG codons as the translation start site (23,24). Nonetheless, three yeast genes, GRS1 (one of the two glycyl-tRNA synthetase (GlyRS) genes in S. cerevisiae) (11), ALA1 (the only alanyl-tRNA synthetase (AlaRS) gene in S. cerevisiae) (12), and CARP2A (the gene coding for acidic ribosomal protein P2A in Candida albicans) (25), have recently been reported to use naturally occurring non-AUG triplets as translation initiators. Moreover, a very recent study suggested that the translational efficiency of non-AUG initiation codons is significantly affected (up to 32-fold) by nucleotides at its relative Ϫ3 to Ϫ1 position, and, to a lesser extent, ϩ4 (26).…”

mentioning

confidence: 99%

Translational Efficiency of Redundant ACG Initiator Codons Is Enhanced by a Favorable Sequence Context and Remedial Initiation

Chen

Yen

et al. 2009

Earlier studies showed that the redundancy of ACG initiation codons enhanced the efficiency of translation initiation by 3-to 6-fold. Evidence presented here shows that this "redundancy effect" can be attributed to a favorable sequence context and, to a lesser extent, remedial initiation. In the case of redundant ACG initiator codons, the second ACG not only acts as a remedial initiation site for scanning ribosomes that skip the first ACG but also enhances the activity of the preceding initiator by providing a preferable "A" at its relative ؉4 position. Hence, nonsuccessive ACG codons can be as effective as successive ACG codons in initiation, if positioned within a similar context. In contrast, redundant GUG initiation codons (GUG/GUG) bear an unfavorable "G" nucleotide at both the ؉4 and ؊3 positions relative to the first and second GUGs, respectively, such that redundant GUG codons act more poorly as translation initiation sites than does a single GUG with a favorable "A" nucleotide in the ؉4 position (ϳ2.5-fold). Thus, the sequence context plays a much more important role than remedial initiation in modulating the efficiency of translational initiation from redundant non-AUG codons.