Selective inhibition of splicing at the avian sarcoma virus src 3 ' splicesite by direct-repeat posttranscriptional cis elements

The direct-repeat elements (dr1) of avian sarcoma virus (ASV) and leukosis virus have the properties of constitutive transport elements (CTEs), which facilitate cytoplasmic accumulation of unspliced RNA, It is thought that th ese elements represent binding sites for cellular factors. Previous studies have indicated that in the context of the avian sarcoma virus genome, prec ise deletion of both ASV dr1 elements results in a very low level of virus replication, This is characterized by a decreased cytoplasmic accumulation of unspliced RNA and a selective increase in spliced src mRNA, Deletion of either the upstream or downstream dr1 results in a delayed-replication phen otype, To determine if the same regions of the dr1 mediate inhibition of sr c splicing and unspliced RNA transport, point mutations in the upstream and downstream elements were studied. In the context of viral genomes with sin gle dr1 elements, the effects of the mutations on virus replication and inc reases in src splicing closely paralleled the effects of the mutations on C TE activity, For mutants strongly affecting CTE activity and splicing, unsp liced RNA but not spliced RNA turned over in the nucleus more rapidly than wild-type RNA. In the context of wild-type virus containing two dr1 element s, mutations of either element that strongly affect CTE activity caused a m arked delay of virus replication and a selective increase in src splicing. However, the turnover of the mutant unspliced RNA as well as the spliced mR NA species did not differ significantly from that of the wild type. These r esults suggest the dr1 elements in ASV act to selectively inhibit src splic ing and that both elements contribute to the fitness of the wild-type virus , However, a single dr1 element is sufficient to stabilize unspliced RNA.