Transcriptional Regulation and Biological Function of the RNA
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Regulation and biological function of the RNA ligase RtcB in Escherichia coli Georgia Mary Hann Centre for Bacterial Cell Biology Institute for Cell and Molecular Biosciences Thesis submitted for the degree Doctor of Philosophy August 2017 Abstract All organisms contain RNA ligases for the modification or repair of RNA molecules. The RtcB family have a novel 3ʹ-5ʹ RNA ligase activity and are conserved in all three domains of life. Metazoan homologues of RtcB have essential roles in tRNA maturation and the unfolded protein response. In E. coli, expression of RtcB from the σ54-dependent rtcBA operon is regulated by transcriptional activator RtcR. Neither the signals that induce rtcB expression nor the natural substrates of RtcB in E. coli are known. The objective of this project is to understand the regulation of rtcB expression by RtcR and the biological function of RtcB in E. coli. In this study, we confirm the roles of RtcR and σ54 in rtcB transcription in E. coli. Using a series of truncated rtcBA promoter fusions with the reporter gene lacZ, we have identified an inverted repeat that we hypothesise is the RtcR binding site in the promoter of rtcB. Mutation of the inverted repeat supports its involvement in RtcR binding. Using a similar approach, we have identified a possible σ70 promoter responsible for transcription of rtcR and provide evidence of negative autoregulation of rtcR. Moreover, we also demonstrate for the first time the requirement of IHF for RtcR-dependent transcription of rtcB and confirm the putative binding site of IHF in the rtcBA promoter in vivo and in vitro. Using a combination of genetic screening with a chromosomal rtcBA-lacZ fusion and quantitative PCR analysis, we have uncovered several conditions where transcription of rtcB is activated in E. coli. Expression of Rof, an inhibitor of Rho transcription termination, induced rtcB transcription in an RtcR-dependent manner, indicating a possible link between RtcB function and disruption of transcription termination. Furthermore, treatment of cells with chloramphenicol and induction of endoribonuclease MazF increased rtcB transcription, suggesting that RtcB may respond to the translational status of the cell. We have used these conditions to perform CRAC (crosslinking and analysis of cDNA) of RtcB, in order to identify substrate RNAs that interact with RtcB in vivo. Many interactions between RtcB and RNAs were identified including rRNAs, tRNAs, sRNAs and mRNAs and several candidates were further analysed by qPCR. Taken together, the data suggests that rtcB has features typical of a σ54-dependent promoter and that RtcB expression can be activated in a variety of stress conditions, thereby allowing interaction with a range of cellular RNAs in E. coli. Nevertheless, the specific signal(s) detected by RtcR and the biological function of RtcB in E. coli remain elusive. i Acknowledgements I would like to thank my supervisor Kenn Gerdes for the opportunity to work on this project and for help and guidance throughout. I also thank Kristoffer Winther for useful scientific discussions and help with experimental procedures. I am grateful to Kathryn Turnbull for many useful discussions, scientific and otherwise. I also acknowledge and thank past and present members of the Gerdes group for their support, technical expertise and suggestions. I am grateful to members of the Centre for Bacterial Cell Biology, Newcastle University and the Centre for Bacterial Stress Response and Persistence, University of Copenhagen for providing a welcoming and stimulating work environment. I thank Michael Sørensen and his group (University of Copenhagen) for an interesting collaboration. Finally, I thank Mick Hollier, Susan Hann and David Hann for their constant love, encouragement and moral support. ii Contents Abstract .................................................................................................................................... i Acknowledgements ................................................................................................................. ii Contents ................................................................................................................................. iii List of figures ........................................................................................................................ vii List of tables ........................................................................................................................... ix Abbreviations .......................................................................................................................... x Chapter 1 Introduction ....................................................................................................... 1 1.1 Overview ...................................................................................................................... 1 1.2 Promoter recognition in bacterial transcription ........................................................... 2 1.2.1 Bacterial transcription and the RNA polymerase holoenzyme ............................ 2 1.2.2 The σ factor of RNA polymerase holoenzyme ..................................................... 3 1.3 Sigma 54-dependent transcription ............................................................................... 5 1.3.1 Mode of transcription initiation by σ54-associated RNAP .................................... 5 1.3.2 Bacterial enhancer binding proteins ..................................................................... 8 1.3.2.1 ATPase domain of bEBPs ............................................................................. 8 1.3.2.2 Regulatory domain of bEBPs ...................................................................... 10 1.3.2.3 DNA binding domain of bEBPs .................................................................. 11 1.3.3 The role of integration host factor in σ54-dependent transcription ..................... 13 1.3.4 The σ54 regulon ................................................................................................... 14 1.4 Sigma 54-dependent transcription of the rtcBA operon in E. coli ............................. 14 1.4.1 Features of the rtcBA promoter .......................................................................... 14 1.4.2 RtcR, the bEBP of the rtcBA operon .................................................................. 15 1.5 The RNA ligase RtcB ................................................................................................ 17 1.5.1 Types and functions of RNA ligases .................................................................. 17 1.5.1.1 5ʹ-3ʹ RNA ligases ........................................................................................ 17 1.5.1.2 2ʹ-5ʹ RNA ligases ........................................................................................ 19 1.5.1.3 3ʹ-5ʹ RNA ligases ........................................................................................ 20 iii 1.5.2 Mechanism of action of RtcB ............................................................................. 20 1.5.3 Additional activities of RtcB in DNA modification ........................................... 24 1.6 Function and conservation of RtcB ........................................................................... 24 1.6.1 Members of the RtcB family are present in all domains of life ......................... 24 1.6.2 Function in eukaryotes and archaea.................................................................... 25 1.6.2.1 tRNA maturation ......................................................................................... 25 1.6.2.2 The unfolded protein response .................................................................... 27 1.6.2.3 Other indications of function ...................................................................... 29 1.6.3 Function in bacteria ............................................................................................ 32 1.6.4 Genes commonly associated with rtcB in bacteria ............................................. 34 1.6.4.1 The RNA cyclase RtcA ............................................................................... 34 1.6.4.2 Ro, an RNA scavenger ................................................................................ 36 1.6.4.3 The RtcB cofactor archease......................................................................... 37 1.7 Aims of this work ...................................................................................................... 38 Chapter 2 Materials and methods .................................................................................... 39 2.1 Bacterial strains, media and growth conditions ......................................................... 39 2.2 Standard DNA procedures ......................................................................................... 39 2.3 Strain construction ..................................................................................................... 46 2.4 Plasmid construction .................................................................................................. 50 2.4.1 Transcriptional fusions ....................................................................................... 50 2.4.2 Overexpression of rtc genes ............................................................................... 55 2.4.3 Expression of tagged rtc genes ........................................................................... 56 2.4.4 Others ................................................................................................................