TRANSLATIONAL INHIBITION MEDIATED BY A SHORT UPSTREAM OPEN READING FRAME IN THE HUMAN CYTOMEGALOVIRUS GPUL4 (GP48) TRANSCRIPT

The human cytomegalovirus (CMV) virion glycoprotein gpULA (gp48) gene expresses a transcript that contains three AUG codons upstream from th e one used to initiate synthesis of the gp48 protein. Previously we re ported that the second of these AUG codons, AUG2, was necessary but in sufficient for inhibition of downstream translation (M. Schleiss, C. R . Degnin, and A. P. Geballe, J. Virol. 65:6782-6789, 1991). We now dem onstrate that the coding information of the upstream open reading fram e initiated by AUG2 (uORF2) is critical for the inhibitory signal. Sev eral missense mutations, particularly those involving the carboxy-term inal codons of uORF2, inactivate the inhibitory signal, while mutation s that preserve the coding content of uORF2 uniformly retain the inhib itory signal. The uORF2 termination codon is essential for inhibition, but leader sequences further downstream are not critical. Conservatio n of uORF2 among clinical strains of CMV suggests that uORF2 provides an important function in the CMV infectious cycle. Although these resu lts indicate that the peptide product of uORF2 mediates the inhibitory effect, we demonstrate that the uORF2 signal acts only in cis, and we propose a model of inhibition by the gp48 uORF2 signal.