f1ab

f1ab, commonly written ORF1ab in coronavirus research, refers to the replicase polyprotein encoded by the ORF1ab region of coronavirus genomes. It is produced when the ribosome undergoes a -1 ribosomal frameshift during translation of the ORF1a/ORF1b region, enabling the production of a longer polyprotein called pp1ab. The ORF1ab region spans a large portion of the 5' to 3' genome and, in SARS-CoV-2, comprises about two-thirds of the genome length.

After translation, the pp1ab polyprotein is cleaved by virally encoded proteases into 16 non-structural proteins (nsp1–nsp16). These nsps assemble into the replication-transcription complex responsible for viral RNA synthesis and processing. Key enzymes within this set include the RNA-dependent RNA polymerase (nsp12), the helicase (nsp13), and proofreading and capping enzymes such as nsp14 (exonuclease) and nsp16 (2'-O-methyltransferase). Proteolytic processing is carried out by the papain-like protease activity in nsp3 (PLpro) and the 3C-like protease activity in nsp5 (3CLpro).

The f1ab/ORF1ab region is highly conserved among coronaviruses and is essential for viral replication, making it a central focus of studies on coronavirus biology and antiviral targets. Its products form the core machinery that supports replication and transcription of the viral genome, underscoring the region’s critical role in the viral life cycle.