Positioning of the translation initiation complex on mRNAs requires interaction between the anticodon of initiator Met-tRNA, associated with eIF2-GTP and 40S ribosomal subunit, and the cognate start codon of the mRNA. We show that an internal ribosome entry site located in the genome of cricket paralysis virus can form 80S ribosomes without initiator Met-tRNA, eIF2, or GTP hydrolysis, with a CCU triplet in the ribosomal P site and a GCU triplet in the A site. P-site mutagenesis revealed that the P site was not decoded, and protein sequence analysis showed that translation initiates at the triplet in the A site. Translational initiation from the A site of the ribosome suggests that the repertoire of translated open reading frames in eukaryotic mRNAs may be greater than anticipated.

Cricket paralysis virus is a member of a group of insect picorna-like viruses. Cloning and sequencing of the single plus-strand RNA genome revealed the presence of two nonoverlapping open reading frames, ORF1 and ORF2, that encode the nonstructural and structural proteins, respectively. We show that each ORF is preceded by one internal ribosome entry site (IRES). The intergenic IRES is located 6,024 nucleotides from the 5 end of the viral RNA and is more active than the IRES located at the 5 end of the RNA, providing a mechanistic explanation for the increased abundance of structural proteins relative to nonstructural proteins in infected cells. Mutational analysis of this intergenic-region IRES revealed that ORF2 begins with a noncognate CCU triplet. Complementarity of this CCU triplet with sequences in the IRES is important for IRES function, pointing to an involvement of RNA-RNA interactions in translation initiation. Thus, the cricket paralysis virus genome is an example of a naturally occurring, functionally dicistronic eukaryotic mRNA whose translation is controlled by two IRES elements located at the 5 end and in the middle of the mRNA. This finding argues that eukaryotic mRNAs can express multiple proteins not only by polyprotein processing, reinitiation and frameshifting but also by using multiple IRES elements.

nanos mRNA, which encodes the localized component of the Drosophila posterior body patterning determinant, is normally translated only at the posterior pole of the embryo, where the mRNA is concentrated. Here we identify two similar cis-acting sequences in the nanos mRNA 3' untranslated region that mediate translational repression. These sequences bind an embryonic protein of 135 kD, smaug, and we refer to them as smaug recognition elements (SREs). Analysis of point mutations in the SREs reveals a strong correlation between smaug binding and translational repression; mutants unable to bind smaug in vitro are not repressed translationally in vivo, whereas mutants that do bind smaug remain repressed translationally. These results strongly suggest that smaug acts in translational repression of unlocalized nanos mRNA. Translational repression is essential, as embryos expressing a nanos mRNA with mutated SREs develop with anterior body patterning defects and die, despite correct localization of the RNA.

A longitudinal cohort study of older adults with chronic obstructive pulmonary disease (COPD) who were stratified by FEV(1) at enrollment was done to define the etiology, frequency, severity, and medical-care impact of respiratory tract viral infections (RTVIs). Controls consisted of a group of subjects of comparable age with the patients. RTVIs were documented in 44% of observed acute respiratory illnesses in control subjects and in 27% of COPD subjects, who were followed for mean periods of 35 and 26 mo, respectively. In this heavily influenza-vaccinated cohort ( approximately 90% vaccinated each year), picornaviruses, parainfluenza viruses, and coronaviruses were most commonly identified. Mean time to return to clinical baseline was approximately 2 wk in each group. Control and COPD subjects with mild airways obstruction (baseline FEV(1) >/= 50% predicted) had few emergency-center visits or hospitalizations. Approximately half of COPD subjects with moderate/severe COPD (baseline FEV(1) < 50% predicted) had at least one emergency-center visit and/or hospitalization for acute respiratory illness. RTVIs were documented in 23% of hospitalizations and in 45% of patients admitted between December and March. RTVIs have a major impact on utilization of health care resources for COPD patients with moderate/severe airways obstruction.

No abstract