Lambda-Cro protein

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

CA runs increase DNA flexibility in the complex of lambda Cro protein with the OR3 site. Biochemistry, 32, 4121-4127. 

Several fusions have been made at the 5 end of the luciferase gene without loss of enzymatic activity. The smallest fusion (in plasmid pKWlOl) is one which resulted in seven amino acids of the lambda cro protein being fused to a truncated luciferase missing the first six amino acids (24). An intermediate size fusion, resulted in twenty amino acids of cro protein fused onto the end of a truncated luciferase (pKWlOS)... 

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.

Superficially, the lambda repressor protein is very different from lambda Cro. The polypeptide chain is much larger, 236 amino acids, and is composed of two domains that can be released as separate fragments by mild proteolysis. In repressor the domain responsible for dimerization is separate from the... 

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. 

Both lambda Cro and repressor proteins have a specific DNA-binding motif... 

The presence of this common helix-turn-helix motif poised for DNA binding in lambda Cro and repressor provided considerable stimulus for further genetic and structural studies of these and other procaryotic DNA-binding proteins. All the results essentially supported the proposed mode of binding between these regulator proteins and DNA. 

The three-dimensional strucmres of Cro and of the lambda repressor protein have been determined by x-ray crystallography, and models for their binding and effecting the above-described molecular and genetic events have been proposed and tested. Both bind to DNA using hehx-turn-helix DNA binding domain motifs (see below). 

R. G. Brennan, S. L. Roderick, Y. Takeda, and B. W. Matthews, Protein-DNA conformational changes in the crystal structure of a lambda Cro-operator complex. Proc. Natl. Acad. Sci. USA, 87 (1990), 8165. 

Observation of the unfolding of a protein 313 Observation of the unfolding of a protein, e.g. lambda Cro 314... 

Figure 8.3 The DNA-binding protein Cro from bacteriophage lambda contains 66 amino acid residues that fold into three a helices and three P strands, (a) A plot of the Ca positions of the first 62 residues of the polypeptide chain. The four C-terminal residues are not visible in the electron density map. (b) A schematic diagram of the subunit structure. a helices 2 and 3 that form the helix-turn-helix motif ate colored blue and red, respectively. The view is different from that in (a), [(a) Adapted from W.F. Anderson et al., Nature 290 754-758, 1981. (b) Adapted from D. Ohlendorf et al., /. Mol. Biol. 169 757-769, 1983.]...

Once the lambda repressor has been inactivated, the positive and negative control exerted by this repressor are abolished, and new transcriptional events can be initiated. The most important transcriptional event is that involved in the synthesis of another lambda protein called Cro. coded by a gene called cro. The gene cro is located almost adjacent to the gene cl which codes for lambda repressor. 

---