A stable upstream stem-loop structure enhances selection of the first 5'-ORF-AUG as a main start codon for translation initiation of human ACAT1 mRNA.

State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, the Chinese Academy of Sciences.

Human ACAT 1 cDNA K1 was first cloned and functionally expressed in 1993. There are two adjacent in-frame AUG codons, AUG(1397-1399) and AUG(1415-1417), at 5'-terminus of the open reading frame (ORF, nt 1397-3049) of human ACAT1 mRNA corresponding to cDNA K1. In current work, these two adjacent inframe AUGs at 5'-terminus of the predicted ORF (5'-ORF-AUGs) as start codons for translation initiation of human ACAT1 mRNA were characterized in detail. Codon mutations indicated that both of these two adjacent 5'-ORF-AUGs can be selected as start codons but the first 5'-ORF-AUG(1397-1399) is a main start codon consistent with that of the predicted ORF of human ACAT1 mRNA. Further deletion and mutation analyses demonstrated that a stable upstream stem-loop structure enhanced the selection of the first 5'-ORF-AUG(1397 -1399) as a main start codon, in addition to upstream nucleotide A in the -3 position, which is a key site of Kozak sequence. In addition, result of ACAT1 enzymatic activity assay showed no obvious difference between these two ACAT1 proteins respectively initiated from the two adjacent 5'-ORF-AUGs. This work showed that a stable upstream stem-loop structure could modulate the start codon selection during translation initiation of mRNAs that contain adjacent multi-5'-ORF-AUGs.

PMID: 15253151 [PubMed - indexed for MEDLINE]