The Transcription Start Site (TSS) is the specific location on the DNA where RNA polymerase begins synthesizing RNA. It marks the first nucleotide that is transcribed into RNA during gene expression. The TSS is typically labeled as +1 in nucleotide numbering, with upstream bases given negative numbers (e.g., -1, -10, -25).
Key Characteristics:
It is the first base of the RNA transcript.
Located downstream of the core promoter elements, such as the TATA box.
Surrounded by regulatory sequences like the initiator (Inr) element, which helps direct transcription.
The DNA at the TSS is usually part of an open complex—unwound to allow transcription to begin.
Function:
Function
Description
Initiation point
Defines where RNA polymerase starts RNA synthesis
Orientation reference
Sets the direction for transcription (5′ to 3′ of the RNA strand)
Gene regulation
The position and usage of TSSs can be modulated to control gene expression
TSS and Promoters:
The TSS lies within or just downstream of the core promoter.
Elements like the TATA box (~25 bp upstream) and Inr (at or near +1) help recruit RNA polymerase II to the correct position.
Biological Relevance:
Some genes have multiple TSSs, allowing alternative transcripts from the same gene.
Mutations near the TSS can alter transcription efficiency or start position, affecting protein expression.
TSS mapping (e.g., via techniques like CAGE) is used to study gene regulation, cell type-specific expression, and promoter usage.
Summary Table:
Feature
Description
Location
First base of RNA transcript (position +1)
Function
Marks where transcription begins
Associated With
Core promoter, Inr element, TATA box
Importance
Defines transcription direction and start point
Clinical Relevance
Mutations can affect gene expression and cause disease
