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| Structure of CII, a transcriptional activator of bacteriophage lambda, with a novel quaternary assoc | |
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Ajit B. Datta (1) Santosh Panjikar (2) Manfred Weiss (2) Pinak Chakrabarti (1)
and Pradeep Parrack (1)
(1) Department of Biochemistry, Bose Institute , Calcutta 700 054, India
(2) EMBL Hamburg Outstation, c/o DESY, Notkestrasse 85, 22603 Hamburg, Germany
The transcriptional activator protein CII of coliphage lambda is a key element in
the decision making switch that governs phage development in one of its two
alternate pathways, viz. lytic or lysogenic. It is small protein (97 amino acids
per subunit) and exists as a tetramer in the native state. CII is a highly unstable
protein with a very small half-life in vivo. This in vivo instability is essential
for its function and probably accounts for the fact that although the importance
of CII in the lysis-lysogeny switch was identified some twenty years ago, there
has been little progress in unravelling the 3-dimensional structure.
The structure has been solved using MAD data involving Se-methionyl derivative,
collected first at Spring-8 and later at DESY. The structure has been refined to
a free R-value of 28% with 2.6 Ã
data. About 17 residues at the C-terminal end of
all the subunits could not be seen in the electron density. The structure is
essentially helical. An interesting quaternary structure is observed for the
protein, in which four chains are mainly in contact through the C-terminal
helix. While the individual subunits in homo-oligomeric proteins normally
assume the same tertiary structure, in CII there are significant differences
between them. The operator site of other transcription factors with
helix-turn-helix (HTH) motif normally has an approximate two-fold symmetry
(i.e., palindromic sequence); however, CII is unique in the sense that it
binds a direct repeat sequence (T-T-G-C-N6-T-T-G-C, N6 representing any 6
bases) and the structure obtained provides an explanation for this.
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