Final answer:
A scientist could perform a reporter assay with site-directed mutagenesis to support the hypothesis that a transcription regulation site acts as a repressor by comparing the activity levels of reporter gene expression between the wild-type and a mutated construct.
Step-by-step explanation:
To test the hypothesis that a potential transcription regulation site 300bp downstream of a gene acts as a repressor, the scientist could use a reporter assay combined with site-directed mutagenesis. This experiment involves creating two versions of a reporter construct. One will be the wild-type with the intact potential repressor site and the other, a mutated version where the repressor site has been altered or deleted. The constructs are then separately transfected into suitable eukaryotic cells followed by measuring the reporter gene's activity. If the intact site acts as a repressor, the reporter activity should be lower in cells with the wild-type construct compared to the mutated construct.
For more precise understanding, the scientist could also introduce a known repressor protein into the cell and observe whether it binds to the regulatory site, resulting in reduced transcriptional activity. This can be done using electrophoretic mobility shift assays (EMSAs) or chromatin immunoprecipitation (ChIP) assays.
If the hypothesis is correct, and the site functions as a repressor, altering or removing it would result in increased transcription of the reporter gene. Conversely, binding of a repressor protein to the site would decrease reporter gene expression compared to a control. These outcomes would support the hypothesis that the identified site functions as a repressor, influencing the transcription of the gene located upstream.