Figure 1. Test queries: lac operon and Lsr system.

A. The lac operon is composed of beta-galactosidase (LacZ), the lactose importer (LacY), and a beta-galactoside transacetylase (LacA). Upstream of the operon, the operon repressor (LacI) is expressed in a co-directional orientation. The primary function of the lac system is as a regulated importer/processing unit. Lactose brought in through the permease LacY is converted into allolactose or hydrolyzed into glucose and beta-galactose. Both reactions are catalyzed by LacZ. Allolactose then acts to release the repression of the system by LacI. B. The Lsr system is composed of two divergent operons. One operon consists of an AI-2 kinase and a system repressor. The other operon consists of an AI-2 transporter and phospho-AI2 processing genes. Contextual system behavior is partly governed by distinctly regulated parts including an alternative importer [23], an exporter [33], and the AI-2 synthase gene. Relative to the canonical lac system, the Lsr system is complicated by the fact that the cell synthesizes,exports, and imports AI-2, and by the negative regulation associated with the divergently arranged structure. AI-2 exported by a mechanism involving YdgG traverses the outer membrane through a porin and enters the periplasmic space. Through the ABC-type importer, LsrACDB, AI-2 is then transported back into the cytosol. Once there, AI-2 is phosphorylated by LsrK. This phosphorylated form (AI-2-P) derepresses the Lsr system and is catabolized by LsrF and LsrG into separate downstream products.