Repressor of temperate mycobacteriophage LI harbors a stable C-terminal domain and binds to different asymmetric operator DNAs with variable affinity

Background: Lysogenic mode of life cycle of a temperate bacteriophage is generally maintained by a protein called 'repressor'. Repressor proteins of temperate lambdoid phages bind to a few symmetric operator DNAs in order to regulate their gene expression. In contrast, repressor molecules of temperate mycobacteriophages and some other phages bind to multiple asymmetric operator DNAs. Very little is known at present about the structure- function relationship of any mycobacteriophage repressor. Results: Using highly purified repressor (CI) of temperate mycobacteriophage LI, we have demonstrated here that LI CI harbors an N-terminal domain (NTD) and a C- terminal domain (CTD) which are separated by a small hinge region. Interestingly, CTD is more compact than NTD at 25 degrees C. Both CTD and CI contain significant amount of alpha-helix at 30 degrees C but unfold partly at 42 degrees C. At nearly 200 nM concentration, both proteins form appreciable amount of dimers in solution. Additional studies reveal that CI binds to O-64 and OL types of asymmetric operators of LI with variable affinity at 25 degrees C. Interestingly, repressor - operator interaction is affected drastically at 42 degrees C. The conformational change of CI is most possibly responsible for its reduced operator binding affinity at 42 degrees C. Conclusion: Repressors encoded by mycobacteriophages differ significantly from the repressor proteins of. and related phages at functional level but at structural level they are nearly similar.