Figure 1. Regulation at the transcriptional level. (a) Binding of σ factor to the core enzyme gives active RNA-polymerase. (b) Regulation of the lac operon. i) the repressor gene is transcribed, the repressor R is synthesized and binds in the operator region, so there is no transcription. ii) if lactose is available, small amounts are transported into the cells (basic levels of LacZ are present). Allolactose formed in small amounts binds to the repressor that dissociates from the operator, allowing weak transcription of the lac operon. iii) if glucose is not available, the c-AMP level in the cells increases, and c-AMP binds to CRP that now functions as a transcription activator. lacZ codes for β-galactosidase, lacY for a lactose symporter driven by a proton gradient and lacA for a transacetylase.

Figure 2. Regulation of transcription of the trp-operon by attenuation. i) tryptophan is limiting, the two trp codons (ϒϒ) in the leader peptide cause movement of the ribosomes to be attenuated, but they finally synthesize the leader peptide, the trp operon is transcribed by the RNA polymerase (•), and the ribosomes proceed with the synthesis of the enzymes. ii) tryptophan is in excess, the RNA polymerase is followed by the ribosomes, and a secondary structure of the m-RNA is formed that blocks transcription and translation; large oval, ribosome; small oval, RNA polymerase.

Figure 3. Expression of the nif genes in K. pneumoniae under nitrogen limitation and anaerobic conditions. NifL-NifA is the inhibitory complex, and NifA is the activator. Sequestration of the complex occurs by the modified nitrogen sensory protein GlnK (low ammonia concentration) and by reduction of the FAD-NifL to FADH ₂-NifL (low oxygen concentration). FADH ₂-NifL is bound at the cytoplasmic membrane; RNAP, RNA polymerase (R. Thummer et al., J. Biol. Chem.; 282, 12517–12525; 2007, modified).