Poly(A)-driven and poly(A)-assisted termination: two different modes of poly(A)-dependent transcription termination

Area of Science:

• Molecular Biology
• Gene Regulation
• Transcription Termination

Background:

• Polyadenylation signals (poly(A) sites) are crucial for messenger RNA processing and transcription termination.
• Distinct mechanisms of transcription termination exist, varying across different gene systems and organisms.

Purpose of the Study:

• To investigate the DNA elements mediating transcription termination downstream of chicken beta-globin gene poly(A) sites.
• To compare the mechanisms of poly(A)-driven termination in beta-globin genes with other known termination systems, such as simian virus 40 (SV40).
• To explore the relationship between promoter-proximal premature termination and poly(A)-driven termination.

Main Methods:

• Run-on transcription analysis using a vector with two G-free cassettes.
• Systematic replacement of native 3'-flanking DNA with bacterial DNA.
• Analysis of termination efficiency across different DNA sequences and sequence contexts.

Main Results:

• No unique DNA element was identified as essential for termination downstream of beta-globin poly(A) sites; bacterial DNA also supported termination.
• Termination efficiency was primarily dependent on a functional poly(A) signal, irrespective of downstream DNA sequence.
• Poly(A)-driven termination and promoter-proximal premature termination efficiencies varied similarly across different DNA sequences, suggesting a common underlying mechanism related to polymerase processivity.
• The beta-globin and SV40 termination mechanisms differ, with beta-globin termination influenced by upstream enhancing sequences and SV40 termination requiring specific elements.

Conclusions:

• Poly(A)-driven transcription termination in beta-globin genes is largely sequence-independent and primarily dictated by the poly(A) signal.
• The mechanism of poly(A)-driven termination may involve a return to a state of low polymerase processivity.
• Differences in termination mechanisms between beta-globin and SV40 are governed by upstream DNA sequences and context.