A premature termination codon in either exon of minute virus of mice P4 promoter-generated pre-mRNA can inhibit nuclear splicing of the intervening intron in an open reading frame-dependent manner

How premature translation termination codons (PTCs) mediate effects on nuclear RNA processing is unclear. Here we show that a PTC at nucleotide (nt) 385 in the NS1/2 shared exon of P4-generated pre-mRNAs of the autonomous parvovirus minute virus of mice caused a decrease in the accumulated levels of doubly spliced R2 relative to singly spliced R1, although the total accumulated levels of R1 plus R2 remained the same. The effect of this PTC was evident within nuclear RNA, was mediated by a PTC and not a missense transversion mutation at this position, and could be suppressed by improvement of the large intron splice sites and by mutation of the AUG that initiated translation of R1 and R2, In contrast to the PTC at nt 385, the reading frame-dependent effect of the PTC at nt 2018 depended neither on the initiating AUG nor the normal termination codon for NS2; however, it could be suppressed by a single nucleotide deletion mutation in the upstream NS1/2 common exon that shifted the 2018 PTC out of the NS2 open reading frame. This suggested that there was recognition and communication of reading frame between exons on a pre-mRNA in the nucleus prior to or concomitant with splicing.