Self-Assembly of Bacterial Macrofibers: A System Based Upon Hierarchies of Helices
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SELF-ASSEMBLY OF BACTERIAL MACROFIBERS: A SYSTEM BASED UPON HIERARCHIES OF HELICES
NEIL H. MENDELSON University of Arizona, Department of Molecular and Biology, Life Sciences South Building, Tucson, Arizona
Cellular
ABSTRACT Cylindrical-shaped cells of Bacillus subtilis (0.7 by 4 Jm) are the building blocks of macrofibers, highly organized, helically twisted, multifilament structures millimeters to centimeters in length. The forces responsible for self-assembly and the cylinder-helix deformation trace to the assembly of cell wall polymers and restraint of the motions generated by cell growth. An hierarchical relationship exists involving: (i) molecular level events associated with cell surface assembly, which in turn govern, (ii) cellular level events concerned with motions that accompany cell growth, and these in turn drive, (iii) multicellular level events such as the folding and plying of cell filaments to form a mature macrofiber. Cell growth generates new material and engenders twisting of the cell cylinder along a screw axis as it elongates. The helix hand and degree of twist at the cellular level eventually dictate the hand and twist of the mature multifilament macrofiber. Although several different routes can lead to the initiation of macrofiber production, once initiated a repetitive cycle of folding and plying becomes established. The self-assembly proceeds until mechanical and geometrical factors preclude further folding cycles. INTRODUCTION Chirality and helical shape are fundamental aspects of biological materials at levels ranging from molecular to that of the entire organism. This publication deals with an unusual bacterial structure called a macrofiber, which is basically a selfassembling, helically twisted, multifilament fiber made of cells (1). The levels of organization encompassed range from molecular (the polymeric material that comprises the cell wall) to multicellular (the macrofiber itself). The structure of macrofibers and the mechanism of their assembly will be described. A set of generalities will then be enumerated that accounts for the deformation of shape from cylindrical to helical and the construction of a higher order macrofiber. The complex, three-dimensional, mature form achieved can be traced to forces arising during growth as a consequence of the assembly of the cell surface and the response of the cell wall materials to them. Throughout the self-assembly process and, indeed, the entire life span of a macrofiber, the structure is always in a dynamic state of change [2). Its mass increases, its shape constantly changes, and its complexity becomes greater. These changes are anything but chaotic. At all stages of macrofiber morphogenesis, helix hand and twist remain constant [3) and a pattern of dynamic motions becomes established [4] that is reiterated successively until physical properties of the structure hinder further progress. In Mat. Res. Soc. Symp. Proc. Vol. 255. 01992 Materials Research Society
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the macrofiber system, we are dealing, therefore, with a selflimiting
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