Bacillus: Cellular and Molecular Biology
Peter Graumann University of Freiburg, Germany
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Bacillus subtilis is one of the best understood prokaryotes in terms of molecular biology and cell biology. Its superb genetic amenability and relatively large size have provided the powerful tools required to investigate a bacterium from all possible aspects. Recent improvements in fluorescence microscopy techniques have provided novel and amazing insight into the dynamic structure of a single cell organism. Research on B. subtilis has been at the forefront of bacterial molecular biology and cytology, and the organism is a model for differentiation, gene/protein regulation, and cell cycle events in bacteria. The aim of this book is to present an overview of the most recent exciting new research fields, and to provide a picture of the major cytological aspects of a bacterium.
Bacillus subtilis is a ubiquitous soil bacterium that can be easily isolated from soil, using starch as an energy source and a relatively high salt concentration. Ideally, the soil sample is heated up to 100°C for 30 minutes, allowing only for enduring spores to be cultured from the sample. B. subtilis is unique in that it can choose between at least three different genetic programs when nutrients or other resources become scarce, and/or cell density reaches a critical threshold. To survive or adapt to adverse conditions, cells can enter stationary phase, which is characterized by the formation of single motile cells (exponentially growing cells usually grow in chains and are non-motile), differentiate into enduring and metabolically inactive spores, or become competent and take up DNA from the environment for acquisition of new genetic material. In all three cases, strikingly different genetic programs are turned on that guide the cell through the differentiation processes. In addition to this, B. subtilis shows social behavior, in that the cells communicate with each other and form multicellular structures in the form of swarming cells and biofilms. Two-component systems, cascades of different sigma factors, regulatory RNAs, and specific proteolysis of target proteins form an intricate regulatory network, which is beginning to be unraveled not only in terms of specific steps but also in terms of whole complex processes that are connected with each other. Most strikingly, it has become clear that many proteins have specific subcellular addresses in bacterial cells. These findings have established the field of “bacterial cell biology,” and B. subtilis has been a forerunner in this field. Many vital processes are disturbed if proteins lose their specific localization, but the fundamental question of how proteins are targeted and specifically located in a cell lacking intracellular compartments is still unclear for most cases. Therefore, it has become important to also study proteins in terms of their localization within the cell, in addition to analyzing their biochemistry and regulation. This book is intended to show that we are beginning to understand why a bacterial cell functions as a whole entity and in 3D, i.e. how it is spatially organized, and even how bacteria talk to each other or give their life for the sake of the whole community.
In this book, we will take an inside–out approach to Bacillus, starting with duplication of the chromosome, cell cycle and transcriptional regulation, following its actin cytoskeleton underneath the cell membrane, through the membrane, to the cell wall. Finally, we will consider the multiarchitectural processes of biofilm formation and sporulation that embrace many cytological and genetic aspects throughout the cell. As will become apparent to the reader, many chapters overlap in a variety of aspects, which is due to the fact that most processes addressed in the book are interconnected with each other. For example, the specific localization of the replication machinery (Chapter 1) is an important aspect in DNA repair (Chapter 2) and in ordered chromosome segregation (Chapter 3), and also touches aspects of cell division (Chapter 4). Amazingly, the actin-like MreB cytoskeleton (covered in depth in Chapter 8) is essential for viability, for the ordered insertion of cell wall material (Chapter 10) as well as for chromosome segregation (Chapter 3). Thus, a cytoskeletal structure appears to connect and coordinate cell cycle events and rod-shaped cell growth, showing that the cytoskeleton was actually a functional prokaryotic invention. Many processes thought to occur throughout the cell or all over the membrane (Chapter 9) have been found to be spatially confined to discrete regions, which has shed light onto processes such as cell division, replication, cell growth, and sporulation. Even though transcription and translation are coupled in prokaryotes, these processes occur at defined places within the cells (Chapter 5), apparently facilitating ordered chromosome segregation and efficient synthesis of highly expressed proteins and of stable RNA. Regulation of transcription through RNA molecules (Chapter 6) and regulation of protein activity through proteolysis (Chapter 7) have only recently been recognized as major factors affecting bacterial physiology, and are intertwined with the organization of transcription (Chapter 5), sporulation (Chapter 11), and cell division (Chapter 4). Different fields in bacterial cell biology and molecular biology are now growing together, providing the bacterial aficionado with the opportunity to obtain an integral view of the bacterial cell.
Several useful sites on the internet exist that provide tools to study B. subtilis in more depth. Foremost, the Bacillus sequencing consortium has set up a site in which the whole genome of B. subtilis is accessible in a superb way. Also, geno types of many strains with gene deletions are accessible via two websites. Finally, most authors of this book have websites for the interested reader to find further information on the various research areas covered in this book. Read more ....