Figure 4.

Intricate control of restriction in the operons of the Type II R-M systems of PvuII and Esp1396I by controlling C proteins. A small C gene upstream of, and partially overlapping with, R is coexpressed from p_res1, located within the M gene, at low level with R after entry of the self-transmissible PvuII plasmid into a new host, while M is expressed at normal levels from its own two promoters p_mod1 and p_mod2 located within the C gene. A similar C protein operates in Esp1396I, but in this case the genes are convergently transcribed with transcription terminator structures in between, and M is expressed from a promoter under negative control of operator O_R, when engaged by C protein in a manner similar to that of the PvuII system. Briefly, the C protein binds to two palindromic sequences (C boxes) defining operator sites O_R and O_L upstream of the C and R genes. After initial low-level expression of C.PvuII protein from the weak promoter p_res1, positive feedback by high-affinity binding of a C protein dimer to the distal O_L site later stimulates expression from the second promoter p_res, resulting in a leaderless transcript and more C and R protein. The proximal site O_R is a much weaker binding site, but C protein bound at O_L enhances the affinity of O_R for C protein, and at high levels of C protein, the protein-O_R complex downregulates expression of C and R. In this way, C protein is both an activator and negative regulator of its own transcription. In addition, it is a negative regulator of M, which makes sense as overmethylation of DNA may also be harmful to the cell (see text for further details). C.Esp1396I controls O_R, O_L and O_M in a similar manner as described above. In this way, C proteins keep both R-M under control, and have been tentatively identified in >300 R-M systems (Table 2).