Nucleic Acids Research, 2003, Vol. 31, No. 2 608-618
© 2003 Oxford University Press
Identification of a cis-acting element required for shunt-mediated translational initiation of the Sendai virus Y proteins
Department of Genetics and Microbiology, The University of Geneva Medical School (CMU), 1, rue Michel Servet, CH-1211 Geneva 4, Switzerland
*To whom correspondence should be addressed. Tel: +41 22 7025674; Fax: +41 22 7025702; Email: joseph.curran{at}medecine.unige.ch
Shunting is a mechanism that permits translational initiation at internal codons positioned in proximity to a ribosome acceptor sequence. Sendai virus exploits shunting to express a series of proteins that initiate at the fourth and fifth start sites on the P/C mRNA (namely, the Y1 and Y2 proteins, respectively). Shunt-mediated initiation at these sites is codon independent. In an attempt to characterise the acceptor site, an extensive deletion analysis was performed spanning the entire C ORF. Only mutants flanking the Y1/Y2 start sites exhibited altered shunt phenotypes. Some of these significantly enhanced shunting efficiency to the point where the Y1/Y2 proteins were the major translational products of the mRNA. Additionally, removal of a short region just downstream of the Y2 start codon (referred to as 
10) ablated all Y protein initiation via shunting but had no effect on Y expression when the AUG codons were viewed by a scanning ribosome. Point mutations introduced into this 10 sequence severely perturbed shunt-mediated initiation. We also provide evidence that changes in this region of the P/C mRNA may be used to modulate Y protein expression levels in different viral strains.
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