The srsf1 gene encodes serine/arginine-rich splicing factor 1 (SRSF1) that participates
in both constitutive and alternative splicing reactions. This gene possesses two
ultraconserved elements in the 3’ untranslated region (UTR). Skipping of an alternative
intron between the two elements has no effect on the protein-coding sequence, but it generates
a premature stop codon (PTC)-containing mRNA isoform, whose degradation is
considered to depend on nonsense-mediated mRNA decay (NMD). However, several cell
lines (HCT116, RKO, HeLa, and WI38 cells) constitutively expressed significant amounts
of the srsf1 PTC variant. HCT116 cells expressed the PTC variant nearly equivalent to the
major isoform that includes the alternative intron in the 3’ UTR. Inhibition of NMD by silencing
a key effecter UPF1 or by treatment with cycloheximide failed to increase amounts
of the PTC variant in HCT116 cells, and the PTC variant was rather more stable than the
major isoform in the presence of actinomycin D. Our results suggest that the original stop
codon may escape from the NMD surveillance even in skipping of the alternative intron.
The srsf1 gene may produce an alternative splice variant having truncated 3’ UTR to relief
the microRNA- and/or RNA-binding protein-mediated control of translation or degradation
