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Cat. No.: ISCB-500 (for 500pmol)

Cat. No.: ISCB-2500 (for 2500pmol)

Description

IscB (insertion sequences Cas9‑like OrfB) is an RNA‑guided DNA nuclease encoded within the IS200/IS605 transposon family and is considered an evolutionary ancestor of the CRISPR‑Cas9 system. As a member of the OMEGA (Obligate Mobile Element‑Guided Activity) system family, its discovery provides key insights into the molecular evolution of CRISPR‑Cas systems.

IscB is composed of only ~400 amino acids—substantially smaller than canonical Cas9 proteins (1,000–1,400 aa). This compact size results primarily from the absence of the α‑helical recognition lobe (REC lobe) that mediates target recognition in Cas9. Despite its reduced size, IscB retains the core nuclease domains found in Cas9, including the RuvC and HNH domains, which cleave the two strands of target DNA. In addition, IscB contains an N‑terminal PLMP domain and a C‑terminal TAM‑interaction domain (TID).

IscB functions as a ribonucleoprotein complex with its cognate ωRNA, a 200–400 nt non‑coding RNA that is significantly larger than the crRNA:tracrRNA duplex used by Cas9. Cryo‑EM structural studies reveal that the extensive ωRNA scaffold compensates for the missing REC lobe by participating directly in the recognition of the RNA–DNA heteroduplex. This unique structural adaptation represents one of the most distinctive evolutionary innovations of the IscB system.