The right variable region of the African swine fever virus (ASFV) genome is known to contain genes with functions involving virus virulence and host range in swine. A novel open reading frame, ORF I1 1L, which was absent in the non-pathogenic, cell culture-adapted European isolate BA71V, was identified in the pathogenic African isolate Malawi Lil-20/1. The location of I1 1L in the right variable region, together with its absence in BA71V, suggested that I1 1L may have a function in virus virulence and/or host range. Here, we show that the I1 1L gene is highly conserved among pathogenic African, European and Caribbean ASFV field isolates and that it exists either in a short form, encoding a protein of 77-78 amino acids (9.1 kDa) or in a longer form of 93-94 amino acids (11.1 kDa). The presence of two predicted membrane-spanning segments suggests that I1 1L is an integral membrane protein. RT-PCR analysis demonstrated that I1 1L mRNA is expressed late in the virus replication cycle. A recombinant I1 1L gene deletion mutant, ΔI1 1L, was constructed from the ASFV isolate Malawi Lil-20/1 to examine gene function. Deletion of I1 1L did not affect virus replication in swine macrophage cell cultures nor virulence in domestic pigs, indicating that I1 1L is non-essential for growth in vitro and for virus virulence in domestic swine.
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