In order to separate influences of growth rate and promoter activity on protein expression, we introduced the gene encoding green fluorescent protein (GFP), under control of the synthetic isopropyl -D-thiogalactoside (IPTG) inducible hyper-spank promoter34, 35, into the genome of B. subtilis. Since the fluorescent reporter protein does not exert any catalytic activity that impacts cell physiology, it does not influence growth rate, as long as its protein burden remains negligible.
