Background:

The fundamental difference between prokaryotic and eukaryotic translation initiation is that the former uses Shine-Dalgarno (SD) sequence on the mRNA to recruit small ribosome subunit and locate AUG codon or rarely GUC, CUC or UUG codon as a translation start site, while the latter uses eIF4F complex to bind mRNA and locate AUG codon or rarely CUG, UUG or GUC as a start codon by 5` to 3` scanning mechanism. Some of the examples listed below shows the involvement of non-AUG codon as a translation start signal.

In Saccharomyces cerevisiae, GRS1 encodes two isoforms of Glycyl-tRNA synthetases, one initiates from upstream in-frame UUG codon (encoding signal sequence for mitochondrial import) while the regular cytoplasmic version initiates from downstream AUG codon. For the synthesis of Alanine t-RNA synthetase (ALA1) protein a consecutive ACG ACG codon are utilized as an alternate translation start site. It has been reported recently that MHC class-I molecule can load antigenic precursor which is synthesized by using CUG as an initiation codon recognized by leucine tRNA and non-conventional eIF2A translation initiation factor. A genome wide ribosomal profiling analysis of vivo translation study reported that translation initiation from upstream non-AUG codon is widespread in S. cerevisiae under starvation condition. However, the importance and the basic mechanism for these initiations are still unclear. A number of yeast mutants have been identified that are able to initiate at the non-AUG codons. These mutants were designated as Sui (Suppressor of initiation) phenotypes. Sui- phenotypes are novel mutations that causes break down of AUG codon selection fidelity. Thus, Sui- mutants provide an important tool to understand the molecular mechanism of non-AUG codon selection.
The focus of our lab is to understand the molecular mechanism of non-canonical translation initiation processes in Saccharomyces cerevisiae using molecular genetic techniques.

The key questions are as follows:

• How Sui^- mutants recognize UUG codon as a start codon?
• Does 18S rRNA residues play critical role in UUG codon recognition by Sui^- mutant?
• Does cis-acting elements on mRNA plays critical role in non-AUG codon recognition by Sui^- mutants?
• Are there special trans-acting factors involved in non-AUG codon selection?

Currently, There are two main projects running in our lab.

1. To identify the 18S RNA residues in UUG codon utilization (DBT sponsored project)
2. Screening for cis-acting elements in 5`UTR for effective UUG codon recognition in eIF5-G31R background