Prokaryotic transcription – promoter architecture In bacterial (prokaryotic) promoters, the first conserved hexameric sequence (the “-35 element,” usually similar to TTGACA) is typically located at which position relative to the transcription start site (+1)?

Introduction / Context:
Prokaryotic promoters contain two core consensus elements that guide RNA polymerase holoenzyme (via sigma factor) to initiate transcription precisely. Understanding where these hexameric motifs sit relative to +1 is a foundational concept in bacterial gene regulation and biotechnology vector design.

Given Data / Assumptions:

• The transcription start site is designated +1.
• Promoter elements are described by their approximate upstream positions (negative numbers).
• The two canonical hexamers are commonly called the -35 element and the -10 (Pribnow) box.

Concept / Approach:
The -35 element (consensus similar to TTGACA) is positioned about 35 nucleotides upstream of +1 and is crucial for initial recognition by the sigma subunit. The -10 element (consensus similar to TATAAT) lies about 10 nucleotides upstream and facilitates DNA unwinding. These distances are approximate but conserved enough to serve as landmarks in promoter mapping.

Step-by-Step Solution:
Identify the “first hexameric sequence” in standard descriptions: the -35 element.Recall its location: roughly 35 bp upstream of +1.Differentiate from the -10/Pribnow box: ~10 bp upstream.Select the option stating “approximately 35 bases upstream.”

Verification / Alternative check:
Sigma-70 dependent promoters in E. coli are routinely annotated with -35 and -10 elements at these positions; mutational studies confirm their functional placement for efficient initiation.

Why Other Options Are Wrong:

• At +1: this is the start site, not a promoter hexamer.
• Approximately 10 bases upstream: that is the Pribnow (-10) element.
• Approximately 25 bases upstream: typical of the eukaryotic TATA box location, not bacterial.
• Approximately 50 bases upstream: outside the canonical -35 spacing.

Common Pitfalls:
Confusing eukaryotic TATA positioning with bacterial promoter architecture; mixing up the -10 and -35 elements due to both being hexamers.

Final Answer:
Approximately 35 bases upstream of the transcription start site (the -35 region).