Regulation of mRNA 1 expression by the 5′-untranslated region (5′-UTR) of the coronavirus Infectious Bronchitis Virus (IBV)

Abstract

In this report, we show that expression of the coronavirus IBV mRNA1 is regulated by its 5′-UTR. Evidence presented domonstrates that the IBV sequence from nucleotide 1 to 1904 directs very inefficient synthesis of a product of approximately 43 kDa. Deletion of either the first 362 bp or the whole part of the 5′-UTR, however, dramatically increased the expression of the 43 kDa protein species. The mechanisms involved were investigated by two different approaches. Firstly, translation of the same construct in the presence of [3H]-leucine ruled out the possibility that initiation of small reading frames from non-AUG codons located in the 5′-UTR may compete with the authentic AUG initiation codon, and therefore inhibit the expression of ORF 1a. Secondly, expression and deletion analyses of a dicistronic construct showed that translation of the 43 kDa protein was initiated by ribosome internal entry mechanism. These studies suggest that a 'weak' ribosome internal entry signal is located in the 5′-UTR and is involved in the regulation of mRNA1 expression.