Table 1.

The synthetic biology toolbox: common components used in synthetic biological systems

	Component	Function
Transcriptional	Constitutive promoter libraries [19,20]	Provide continuously ON gene expression at pre-determined levels
	Inducible promoters (for example, responsive to tetracycline, IPTG, gaseous acetaldehyde [22], or light [23])	Provide conditional and, in certain cases, titratable gene expression in response to inducer signal
Posttranscriptional	Non-coding regulatory RNAs [28] (such as riboregulators [29,30], ribozyme switches [31,51], and RNAi switches [32,33])	Control protein production levels by regulating mRNA stability or translation initiation in response to molecular input
	Alternative splicing modulators [35]	Control protein production levels or protein activity by regulating alternative splicing of mRNA in response to molecular input
	RNase substrate libraries [80]	Control protein levels through tunable hairpin elements that direct transcript cleavage
Posttranslational	Degradation tags [24,25]	Modulate protein levels by shortening protein half-lives
	Split inteins [26,27]	Provide biosensing and modulate protein activity by conditionally splicing inactive protein fragments together into functional wholes
Structural	Protein [36-39], RNA [40], and DNA [93] scaffolds	Regulate signaling and metabolic pathway flux by controlling the localization and stoichiometry of pathway components and intermediate products

IPTG, isopropyl-β-D-thio-galactoside; RNAi, RNA interference.