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Coronavirus genomes and subgenomes encode six open reading frames (ORFs). [129] In October 2020, researchers discovered a possible overlapping gene named ORF3d, in the SARS‑CoV‑2 genome. It is unknown if the protein produced by ORF3d has any function, but it provokes a strong immune response.
ORF1a is the first open reading frame at the 5' end of the genome. Together ORF1ab occupies about two thirds of the genome, with the remaining third at the 3' end encoding the structural proteins and accessory proteins. [1] [2] [3] It is translated from a 5' capped RNA by cap-dependent translation. [1]
The ORF Finder (Open Reading Frame Finder) [16] is a graphical analysis tool which finds all open reading frames of a selectable minimum size in a user's sequence or in a sequence already in the database. This tool identifies all open reading frames using the standard or alternative genetic codes.
An open reading frame (ORF) is a reading frame that has the potential to be transcribed into RNA and translated into protein. It requires a continuous sequence of DNA which may include a start codon, through a subsequent region which has a length that is a multiple of 3 nucleotides, to a stop codon in the same reading frame.
Out-of-phase overlaps occurs when the shared sequences use different reading frames. This can occur in "phase 1" or "phase 2", depending on whether the reading frames are offset by 1 or 2 nucleotides. Because a codon is three nucleotides long, an offset of three nucleotides is an in-phase, phase 0 frame.
ORF3d is an overlapping gene whose open reading frame overlaps both ORF3a and ORF3c in the SARS-CoV-2 genome. This potentially represents a rare example of all three possible reading frames of the same sequence region encoding functional proteins.
SARS-related coronavirus is a member of the genus Betacoronavirus (group 2) and monotypic of the subgenus Sarbecovirus (subgroup B). [13] Sarbecoviruses, unlike embecoviruses or alphacoronaviruses, have only one papain-like proteinase (PLpro) instead of two in the open reading frame ORF1ab. [14]
[3] [6] [5] [20] Among the most distinctive features of this region in SARS-CoV is the emergence of a 29-nucleotide deletion that split the full-length open reading frame into two smaller ORFs, ORF8a and ORF8b. Viral isolates from early in the SARS epidemic have a full-length, intact ORF8, but the split structure emerged later in the epidemic.