Regulation of Gene Transcription

Division Of Basic Sciences - Nci

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Abstract

Part A. Chromosome structure and function. From our studies of mechanisms of gene regulation, we have previously proposed that the bacterial chromosome (nucleoid) has a condition dependent defined structure that dictates gene expression. HU is the most conserved nucleoid-associated protein in eubacteria, but how it impacts global chromosome organization and gene expression is poorly understood. (i) Using single-molecule tracking, we demonstrate that HU exhibits nonspecific, weak, and transitory interactions with the chromosomal DNA. These interactions are largely mediated by three conserved, surface-exposed lysine residues (triK), which were previously shown to be responsible for nonspecific binding to DNA. The loss of these weak, transitory interactions in a HUa(triKA) mutant results in an over-condensed and mis-segregated nucleoid. Mutating a conserved proline residue (P63A) in the HUa subunit, deleting the HUb subunit, or deleting nucleoid-associated naRNAs, each previously implicated in HU's high-affinity binding to kinked or cruciform DNA, leads to less dramatically altered interacting dynamics of HU compared to the HUa(triKA) mutant, but highly expanded nucleoids. Our results suggest HU plays a dual role in maintaining proper nucleoid volume through its differential interactions with chromosomal DNA. On the one hand, HU compacts the nucleoid through specific DNA structure-binding interactions. On the other hand, it decondenses the nucleoid through many nonspecific, weak, and transitory interactions with the bulk chromosome. Such dynamic interactions may contribute to the viscoelastic properties and fluidity of the bacterial nucleoid to facilitate proper chromosome functions. (ii) By imaging of near-native, unlabeled E. coli cells by soft X-ray tomography, we showed that HU remodels nucleoids by promoting the formation of a dense condensed core surrounded by less condensed isolated domains. Nucleoid remodeling during cell growth and environmental adaptation correlate with pH and ionic strength controlled molecular switch that regulated HUaa dependent intermolecular DNA bundling. Through crystallographic and solution-based studies we show that these effects mechanistically rely on HUaa promiscuity in forming multiple electrostatically driven multimerization interfaces. Changes in DNA bundling consequently affects gene expression globally, likely by constrained DNA supercoiling. Taken together our findings unveil a critical function of HU-DNA interaction in nucleoid remodeling that may serve as a general microbial mechanism for transcriptional regulation to synchronize genetic responses during the cell cycle and adapt to changing environments. (iii) We strived to elucidate how the chromosome's three-dimensional architecture is organized and maintained in bacterial cells. Using fluorescence microscopy techniques, we are probing the organization of the E. coli chromosome by directly visualizing the positions of specifically labeled DNA sites within living cells. Using two orthogonal ParB - parS systems, we were able to simultaneously label two DNA sites in two colors in the same E. coli strain. Our labeling strategy had a fixed locus as a control point in all strains, and additionally had a 'moving' locus that maps the entire chromosome in coarse-grain. Data from our experiments preliminarily suggested that there is a correlation between the linear (genetic) distance separation between DNA sites and their spatial separation. Eventually with the help of computational modeling, we hope to simulate the three-dimensional organization of the chromosome from a wealth of carefully conducted distance measurements between different DNA loci in living cells. (iv) We have additionally demonstrated separate physiological roles of specific and non-specific DNA binding of the histone-like protein HU in E. coli. A manuscript is being prepared. in bacterial physiology from maintenance of chromosome structure to regulation of gene transcription. HU is essential in many pathogens, making it an attractive target for developing anti-microbial therapeutics. A mechanistic understanding of HU DNA binding and its regulation of physiological processes will aid in the design and development of small molecule HU inhibitors. We have used Escherichia coli as a model organism to investigate how HU interacts with chromosomal DNA and regulates various physiological processes. In E. coli, HU binds to DNA in two ways: (i) with low affinity to any DNA (non-specific) through three surface-exposed lysine residues (K3, K18, and K83) that make ionic bonds with DNA phosphates; (ii) with high affinity to contorted DNA of given structures containing a pair of kinks (structure-specific) through conserved proline residues (P63) that mediate specific binding by inducing and/or stabilizing the kinks. We recently demonstrated that HU interacts with chromosomal DNA with rapid association/dissociation kinetics largely through its non-specific binding mediated by the lysine residues. This provides evidence that the overall association of HU to the chromosome is through non-specific binding. Incidentally, HU is essential in many pathogens, making it a target for developing anti-microbial drugs. A mechanistic understanding of HU DNA binding will aid in the design and development of HU inhibitors. Part B. Gene regulation in Bacteriophage Lambda and Gal operon: The current year we have made more progress in our work with phage Lambda. Investigation of RNA Polymerase & CI repressor Interactions . One of the best understood systems in genetic regulatory biology is the so called "genetic switch". This determines the choice the phage-encoded CI repressor makes by binding cooperatively to two tripartite operators, OL (OL1, OL2 & OL3) and OR (OR1, OR2 & OR3), in a defined pattern. Transcription at two lytic promoters, PL and PR, is blocked, while transcription at lysogenic promoter, PRM, is activated and repressed at low CI and high CI concentrations, respectively. The autoregulation of PRM is dependent on the interaction of RNA polymerase (RNAP) binding to the PRM promoter and CI binding to OR2. By using a purified in vitro transcription system, we analyzed the activation complex between RNAP at PRM and CI at OR2 by DNA and protein mutations. We inserted 5-bp or deleted 1-bp DNA between OR2 & OR3 to change the angular orientation and distance between RNAP and CI. We also mutated E34K of CI which interacts with RNAP during the activation of PRM. We obtained unexpected findings. First, a 1-bp DNA deletion of -34A of PRM resulted in the repression of PRM at the same CI concentration for the repression of PL and PR. This repression is depending on DNA looping and the binding of CI to OR2. Second, a 5-bp DNA insertion between the PRM promoter site and OR2 site resulted in the repression of PRM at the same CI concentration for the repression of PL and PR. Third, mutating E34K of CI which is involved in the activation complex resulted in the repression of PRM at the same CI concentration for the repression of PL and PR. Finally, DNA looping enhances PRM activation and repression. Conclusion: Disruption of the activation complex between RNAP at PRM and CI at OR2 by mutating CI or inserting or deleting base pair to change the angular orientation and distance between RNAP and CI led to the repression of PRM. These unexpected results suggest that maybe RNAP is creating negative contacts with CI at OR2 preventing RNAP from escaping and repressing PRM. Future studies are being conducted to *TRUNCATED*