| Chromatin remodeling is fundamentally involved in the expression regulation of genes in all organisms. This is largely due to the transcriptionally repressive chromosome compaction that needs to be regulated so that DNA transactions, such as transcription can occur. By regulating the degree of compaction, architectural proteins play vital roles in both organizing of the chromosomal structure and regulating transcription. Condensation of the chromosomal DNA in bacteria by the histone-like nucleoid structuring protein (H-NS) represses transcription but is modulated by architectural proteins such as integration host factor (IHF) and leucine response regulator (LRP) that can structurally remodel the transcriptionally repressive architecture. However, the mechanisms whereby the architectural proteins regulate the transcriptional repression mediated by H-NS is not well understood. Our research group has recently identified a novel transcription factor, LeuO, which appears to regulate transcription through chromosome architectural changes as well. LeuO regulates the H-NS transcriptional repression chromosome-wide, including at its own gene, leuO. LeuO relieves the H-NS-mediated repression of the leuO gene via a boundary element-like activity. The derepression process plays an important role in the sequential activation of genes in the ilvIH-leuO-leuABCD (promoter relay mechanism). Further exploration of the transcription regulatory activity of LeuO using this model system is likely to provide a paradigm for understanding how transcription factors regulate gene expression via modulating chromosome architecture. In this dissertation research, I demonstrated that LeuO can lead to a series of chromosomal architectural changes that coordinate the participations of H-NS and IHF, as its regulatory partners in the regulation of the promoter relay mechanism. The binding sites of LeuO, H-NS, and IHF are closely positioned or even overlapped in the regulatory region. By looping and wrapping the DNA, LeuO was capable of structurally manipulating the DNA elements and thus coordinating their functions in the promoter relay mechanism. The unraveled mechanism characterizes LeuO as a master architect that regulates the chromosomal attendance of the architectural proteins and thus, instigates the chromosomal modifications, which, in turn, results in the sequential gene activation as described in the promoter relay mechanism. The DNA structural modifications demonstrated in the present study is likely paradigmatic for understanding the mechanisms underlying the chromosomal modifications mediated by the other architectural proteins in both prokaryotes and eukaryotes in general. |
