I was looking at the iGEM Bioregistry of Terminator parts which offer varying degrees of termination efficiency. I am wondering why studies into combinatorial synthesis of genetic circuits for metabolic engineering include terminators as a variable component within their designs, such as this paper by Woodruff et al., 2017? This must come at great complexity cost to overall library size, where I would of assumed one would take the most efficient terminator and fix it within the design. How could terminator strength or sequence effect optimisation of a metabolic pathway in an impactful manner such that this extra complexity becomes worthwhile?
To add to gaspanic's answer, not all terminators work equally well under all conditions. So if terminators weren't characterized in the organism or tissue you are working in the efficiency might be different. Additionally, though rarely people might be looking to design with a leaky terminator mimicking bacterial use of leaky terminators to use one DNA sequence to create two mRNAs of different length.
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