Structural basis of the complete poxvirus transcription initiation process

Area of Science:

• Virology
• Molecular Biology
• Structural Biology

Background:

• Poxviruses express genes in the cytoplasm using a virus-encoded RNA polymerase (vRNAP) and transcription factors.
• Understanding poxviral transcription initiation is crucial for deciphering unique gene expression mechanisms.

Purpose of the Study:

• To elucidate the complete transcription initiation phase of the poxvirus vaccinia using cryo-electron microscopy (cryo-EM).
• To reveal the structural basis of promoter recognition, DNA melting, and transcription directionality by viral transcription factors.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures of transcription complexes.
• Structural analysis of pre-initiation complexes involving vaccinia virus polymerase (vRNAP) and transcription factors.

Main Results:

• The heterodimeric early transcription factor (VETFs/l) forms an arc, spanning the vRNAP cleft and binding promoter DNA.
• VETFI, a TBP-like protein, inserts into the DNA major groove, inducing melting and ensuring promoter recognition and directionality.
• The helicase VETFs facilitates promoter melting, and the Rpo30 subunit's phospho-peptide domain (PPD) is essential for initiation.
• An upstream promoter scrunching mechanism, aided by helicase NPH-I, likely promotes promoter escape.

Conclusions:

• Structural insights into poxviral transcription initiation reveal unique mechanisms involving VETFI, VETFs, and NPH-I.
• These findings illuminate fundamental principles of transcription, potentially applicable beyond poxviruses.
• The study provides a detailed structural understanding of viral gene expression regulation.