ambisense

Ambisense describes a genome organization in which genetic information on a single RNA segment is encoded in both the positive-sense (coding) and negative-sense (template) orientations. In this arrangement, two genes lie on opposite orientations within the same segment, so expression requires transcription from both the viral genome and its complementary antigenome.

Ambisense genome organization is characteristic of certain negative-sense RNA viruses, most notably arenaviruses, and is present in their segmented genomes. In arenaviruses, the S and L segments carry two genes in opposite orientations, separated by a noncoding intergenic region. The gene encoded in the sense orientation is typically transcribed directly from the genomic RNA to produce its mRNA, while the gene encoded in the opposite orientation is transcribed from the antigenome to generate its own mRNA. Transcriptional regulation involves promoter elements at both ends of the segment and termination within the intergenic region.

The ambisense strategy imposes a distinctive replication and transcription cycle. Production of functional proteins from both genes often depends on the synthesis of both genome and antigenome RNA intermediates, with the timing of expression influenced by the replication state of the virus. This dual-polarity arrangement allows a compact genome, enabling two essential genes to be encoded on a single segment.

Ambisense genome organization is a notable example of how viruses maximize coding capacity and regulate gene expression. It is primarily discussed in the context of virology and molecular biology and has relevance to studies of viral replication and antiviral targets.