Cro proteins

SY Chung, S Subbiah. The use of side-chain packing methods m modeling bacteriophage repressor and cro proteins. Pi-otem Sci 4 2300-2309, 1995. 

Figure 8.1 A region of DNA in the related bacteriophages lambda, 434, and P22 that controls the switch for synthesis of new phage particles. Two structural genes are involved in this switch one coding for a repressor protein and one coding for the Cro protein. Between these genes there is an operator region (OR) that contains three protein binding sites—ORl, OR2, and OR3.

Repressor and Cro proteins operate a procaryotic genetic switch region... 

In spite of the absence of the C-terminal domains, the DNA-binding domains of lambda repressor form dimers in the crystals, as a result of interactions between the C-terminal helix number 5 of the two subunits that are somewhat analogous to the interactions of the C-terminal p strand 3 in the Cro protein (Figure 8.7). The two helices pack against each other in the normal way with an inclination of 20° between the helical axes. The structure of the C-terminal domain, which is responsible for the main subunit interactions in the intact repressor, remains unknown. 

The x-ray structure of the complete lambda Cro protein is known The x-ray structure of the DNA-binding... 

The product of the cro gene, the 66-amino-acid, 9 kDa Cro protein, has a single domain but also binds the operator DNA more tightly as a dimer (Figure... 

D). The Cro protein s single domain mediates both operator binding and dimerization. 

The resulting newly synthesized Cro protein also binds to the operator region as a dimer, but its order of preference is opposite to that of repressor (Figure 39—7). That is, Cro binds most t hdy to Or3, but there is no cooperative effect of Cro at 0 3 on the binding of Cro to 0 2. At increasingly higher concentrations of Cro, the protein will bind to Op 2 and evenmally to OrI. 

Figure 39-13. A schematic representation of the three-dimensional structure of Cro protein and its binding to DNA by its helix-turn-helix motif. The Cro monomer consists of three antiparallel p sheets (P1-P3) and three a-helices (a,-a3).The helix-turn-helix motif is formed because the aj and U2 helices are held at about 90 degrees to each other by a turn offour amino acids. The helix of Cro is the DNA recognition surface (shaded). Two monomers associate through the antiparallel P3 sheets to form a dimer that has a twofold axis of symmetry (right). A Cro dimer binds to DNA through its helices, each of which contacts about 5 bp on the same surface of the major groove. The distance between comparable points on the two DNA a-helices is 34 A, which is the distance required for one complete turn of the double helix. (Courtesy of B Mathews.)...

The key to the genetic switch lies in the close proximity of the regulator genes for repressor and cro protein. These two genes are transcribed in opposite directions, beginning at different start points. 

Tidor, B., Helix-capping interaction in A-Cro protein — a free-energy simulation analysis, Proteins Struct. Func. Genet. 1994,19, 310-323. 

Tullius, T.D., and Dombroski, B.A. (1986) Hydroxyl radical footprinting high-resolution information about DNA-protein contacts and application to repressor and Cro protein. PNAS 83, 5469-5473. 

Cro Protein Prevents Buildup of cl Protein during the Lytic Cycle... 

The cro protein and the cl protein bind to exactly the same sites on the DNA. Despite this fact, the cl protein is required for lysogeny, whereas the cro protein is required for lysis. These requirements can be shown with mutants. A cl mutant invariably undergoes lysis, whereas a cro mutant can lysogenize but cannot complete the lytic cycle. This remarkable difference in the behavior of cro and cl results from the fact that although they bind to the same sites, they do so with totally different relative affinities (fig. 30.23). Cro binds preferentially to 0R3 and less strongly to 0R, and Or2. 

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