Genome (VSV) virion and
Mononegavirales genomes All viruses in
Negarnaviricota are negative-sense, single-stranded RNA (−ssRNA) viruses. They have genomes made of RNA, which are single instead of double-stranded. Their genomes are negative sense, meaning that messenger RNA (mRNA) can be synthesized directly from the genome by the viral enzyme RNA-dependent RNA polymerase (RdRp), also called RNA replicase, which is encoded by all −ssRNA viruses. Excluding viruses in the genus
Tenuivirus and some in the family
Chuviridae, all −ssRNA viruses have linear rather than circular genomes, and the genomes may be segmented or non-segmented. All −ssRNA genomes contain
terminal inverted repeats, which are palindromic nucleotide sequences at each end of the genome.
Replication and transcription replication cycle Replication of −ssRNA genomes is executed by RdRp, which initiates replication by binding to a leader sequence on the 3'-end (usually pronounced "three prime end") of the genome. RdRp then uses the negative sense genome as a template to synthesize a positive-sense antigenome. When replicating the antigenome, RdRp first binds to the trailer sequence on the 3'-end of the antigenome. Thereafter, RdRp ignores all transcription signals on the antigenome and synthesizes a copy of the genome while using the antigenome as a template. Replication is executed while the genome is inside the nucleocapsid, and RdRp unveils the capsid and translocates along the genome during replication. As new nucleotide sequences are synthesized by RdRp, capsid proteins are assembled and encapsidate the newly replicated viral RNA. After capping the mRNA, RdRp initiates transcription at a gene start signal and later terminates transcription upon reaching a gene end signal. At the end of transcription, RdRp synthesizes a
polyadenylated tail (poly (A) tail) consisting of hundreds of
adenines in the mRNA's 3-end, which may be done by
stuttering on a sequence of
uracils. After the poly (A) tail is constructed, the mRNA is released by RdRp. In genomes that encode more than one transcribable portion, RdRp can continue scanning to the next start sequence to continue with transcription. Some −ssRNA viruses are
ambisense, meaning that both the negative genomic strand and positive antigenome separately encode different proteins. In order to transcribe ambisense viruses, two rounds of transcription are performed: first, mRNA is produced directly from the genome; second, mRNA is created from the antigenome. All ambisense viruses contain a
hairpin loop structure to stop transcription after the protein's mRNA has been transcribed.
Morphology structure (left);
transmission electron micrograph of
California encephalitis virus (right) Negative-strand RNA viruses contain a
ribonucleoprotein complex composed of the genome and an RdRp attached to each segment of the genome surrounded by a capsid. The capsid is composed of proteins whose folded structure contains five alpha-helices in the
N-terminal lobe (5-H motif) and three alpha-helices in the
C-terminal lobe (3-H motif). Inside the capsid, the genome is sandwiched between these two motifs.
Evolution Genome segmentation is a prominent trait among many −ssRNA viruses, and −ssRNA viruses range from having genomes with one segment, typical for members of the order
Mononegavirales, to genomes with ten segments, as is the case for
Tilapia tilapinevirus. There is no clear trend over time that determines the number of segments, and genome segmentation among −ssRNA viruses appears to be a flexible trait since it has evolved independently on multiple occasions. Most members of the subphylum
Haploviricotina are nonsegmented, whereas segmentation is universal in
Polyploviricotina. ==Phylogenetics==