  Protein synthesis rates are determined, at the translational level, by
properties of the transcript's sequence. The efficiency of an mRNA can be tuned
by varying the ribosome binding sites controlling the recruitment of the
ribosomes, or the codon usage establishing the speed of protein elongation. In
this work we propose transcript length as a further key determinant of
translation efficiency. Based on a physical model that considers the kinetics
of ribosomes advancing on the mRNA and diffusing in its surrounding, as well as
mRNA circularisation and ribosome drop-off, we explain how the transcript
length may play a central role in establishing ribosome recruitment and the
overall translation rate of an mRNA. According to our results, the proximity of
the 3' end to the ribosomal recruitment site of the mRNA could induce a
feedback in the translation process that would favour the recycling of
ribosomes. We also demonstrate how this process may be involved in shaping the
experimental ribosome density-gene length dependence. Finally, we argue that
cells could exploit this mechanism to adjust and balance the usage of its
ribosomal resources.
