How can I tell if regulation is at the transcriptional or translational level?

Question

I was reading a paper, http://www.pnas.org/content/109/8/E471.short, where the authors claim that (e475)

Translation of the TfR (Transferrin Receptor) is regulated through sequences in the 3′ and 5′ UTRs (untranslated regions) of its mRNA. To study the regulation of TfR levels independently of translational control, we measured the levels of TfR-EGFP expressed from a plasmid lacking the regulatory sequences present in the endogenous TfR mRNA (Fig. 6A). Endogenous and GFP-tagged expressed TfR levels were reduced equally in response to Rabenosyn-5 depletion, ruling out an effect of this protein on transcriptional or translational control of TfR expression.

I believe I understand that the effect can not be post-translational, since TfR-GFP has no region for translational regulation and so it cannot be regulated by any translationally active regulating factor.

My question is: Wouldn't the statement above, that Rabenosyn-5 has no effect on protein transcription, require a proof that mRNA levels are not affected by this protein?

Fig. 6A, http://www.pnas.org/content/109/8/E471.figures-only

Answer 1

From the Methods section:

Human TfR in plasmid cDNA was a gift from Tim McGraw (Weill-Cornell Medical College, New York, NY). Human TfR cDNA was subcloned in frame with EGFP in the Clontech pEGFP-N1 vector at the XhoI and BamHI restriction sites.

This TfR-GFP fusion protein does not have the endogenous TfR promoter. So it is not likely to be regulated by any TF that would directly regulate TfR transcription.

So they conclude that Rabenosyn-5 does not specifically regulate TfR expression.