The bacterial chromosome, like its eukaryotic counterparts, must be folded and compacted to fit inside the cellular space allotted to it. The end result of this compaction must not only be a chromosome that """"""""fits"""""""" inside the cell but one that still functions in all of its essential roles. From many studies on the E. coli chromosome, several compaction models have emerged. One of the most significant is the arrangement of the chromosome into 50 to 100 independently supercoiled loops. A second compaction method utilizes the small basic DNA bending proteins (the histone-like proteins) or the large, motor-like protein, MukB. A third method is indicated by the importance of nascent mRNA in maintaining chromosome structure. How mRNA is used in this process is not understood. We have described a system, comprised of three genes, that is used to condense the chromosome. One of the genes, crcA is most likely a regulator of the system. The other two, cspE and crcB, are responsible for the compaction. CspE is a member of the cold shock-like protein family, of which E. coli has nine members. CrcB is predicted to be a small basic protein. It has recently been shown that CspE binds to the nascent mRNA-ribosome-DNA complex and ends up on the protruding end of the mRNA. We are investigating the role of CrcB in this binding and how the complexes formed by these molecules lead to chromosome compaction. We have also found that this system is essential in certain bacterial strains. We are investigating the requirements for this to be an essential system and using the observation to isolate mutations in all of the components. These studies will allow us to define the role of mRNA in compaction of actively transcribed chromatin.
