General remarks of the secondary promoter element regulating the genome replication of paramyxo- and filoviruses

Paramyxo- and filovirus genomes are equipped with bipartite promoters at their 3 ends to initiate RNA synthesis. The two elements, the primary promoter element 1 (PE1) and the secondary promoter element 2 (PE2), are separated by a spacer region that must be precisely a multiple of six nucleotides, indicating these viruses are related to the "rule of six". However, our knowledge of PE2 has been limited to a narrow spectrum of virus species. In this study, a comparative analysis of 1,647 paramyxoviral genomes from a public database revealed that the paramyxovirus PE2 can be clearly categorized into two distinct subcategories: one marked by C repeats at every six bases (exclusive to the subfamily Orthoparamyxovirinae), and another characterized by CG repeats every six bases (observed in the subfamilies Avulavirinae and Rubulavirinae). This unique pattern collectively mirrors the evolutionary lineage of these subfamilies. Furthermore, we showed that the PE2 of the Rubulavirinae, with the exception of mumps virus, serves as part of the gene-coding region. This may be due to the fact that the Rubulavirinae is the only paramyxovirus that cannot propagate without RNA editing occurring. Zaire ebolavirus has eight sequential uracil (U) repeats every six bases within its genomic promoter. We showed that a minimum of four sequential U-containing hexamer repeats is imperative for genome replication. This discovery led to the identification of such quadruplet U-containing hexamer repeats within the genomic and antigenomic promoters of other viruses within the family Filoviridae. SignificanceThe genomic intricacies of paramyxo- and filoviruses are highlighted by the bipartite promoters--PE1 and PE2--at their 3 termini. The spacer region between these elements follows the "rule of six", crucial for genome replication. By a comprehensive analysis of paramyxoviral genome sequences, we identified distinct subcategories of PE2 based on C and CG repeats that were specific to Orthoparamyxovirinae and Avulavirinae/Rubulavirinae, respectively, mirroring their evolutionary lineages. Notably, the PE2 of Rubulavirinae is integrated in the gene-coding region, a unique trait potentially linked to its complete dependence on RNA editing for virus growth. This study also focused on the PE2 sequences in filovirus genomes. In filoviruses, four consecutive U-containing hexamer repeats appeared to be critical for their genome replication.