Stop codon decoding in Candida albicans: from non-standard back to standard.

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• The human pathogen Candida albicans exhibits a non-standard genetic code, translating the leucine-CUG codon as serine.
• This unusual decoding is mediated by a novel serine-tRNA (ser-tRNA(CAG)).
• Previous studies suggested this tRNA caused aberrant mRNA decoding, including stop codon readthrough.

Purpose of the Study:

• To definitively elucidate the in vitro behavior of Candida albicans ser-tRNA(CAG) towards stop codons and other non-standard translational events.
• To distinguish between aberrant decoding caused by CUG codon misreading and stop codon readthrough.
• To investigate the structural impact of leucine-to-serine substitutions on protein synthesis.

Main Methods:

• Utilized cell-free translation systems and reporter gene assays.
• Constructed novel reporter systems specifically lacking CUG codons to isolate translational events.
• Analyzed polypeptide migration patterns on SDS-PAGE to assess structural alterations.

Main Results:

• Demonstrated that aberrant mRNA decoding in vitro is not caused by stop codon readthrough or other non-standard translational events.
• Identified that a single leucine-to-serine substitution at a CUG codon in the Brome Mosaic Virus (BMV) RNA-4 coat protein gene is responsible for aberrant migration.
• Concluded that the ser-tRNA(CAG) exclusively mediates the decoding of the leucine-CUG codon as serine.

Conclusions:

• The sole aberrant translational event mediated by Candida albicans ser-tRNA(CAG) is the decoding of the leucine-CUG codon as serine.
• This amino acid substitution significantly alters the structure of the virion coat protein.
• The findings resolve previous ambiguities regarding non-standard decoding in C. albicans.