Bacterial macrofiber systems [1] will be used to model the self-assembly and morphogenesis of a multicellular state. Rod-shaped cells, 4mm long by 0.7 mm in diameter, are the building blocks of mm length multifilament twisted fibers of a given helix hand and degree of twist. The goal of the profect is to determine the hierarchical relationship between individual cell behavior and morphogenesis of the fiber. Teh origin of forces required for the self-assembly and their magnitudes will also be sought.
Previous work has shown that individual cells grow in length only and normally divide at a given length into two equal daughter cells that separate from one another. If separation is prevented, a constantly elongating chain of cells arises. These multicullular filaments are a necessary intermediate in macrofiber production. Their growth provides the mass needed as well as the forces required for self- assembly. Each individual filament is capable of producing a macrofiber independently if the filament twists as it elongates. Twisting motions are accompanied by bending and writhing which eventually results in the formation of a loop and its winding up into a double standed helix. Multistradedness, results from repetition of the twisting, writhing, loop formation and winding up sequence by double stranded and larger structures. The dynamics and geometry involved are reminiscent of the twist, writhe and supercoiling of DNA and negative twist is a significant feature of both systems [1].
Mendelson, Tabor and Goldstein their postdoctoral associates, graduate students and selected undergraduates (such as Patrick Shipman, a mathematics undergraduate pursuing an undergraduate research project in Mendelson's lab) will pursue experimental and theoretical studies of instabilities that arise in an elastic rod that twists and as it elongates. Attempts will be made to understand writhing in terms of these instabilities and to apply the information gained to explain macrofiber behavior using the extensive mathematical results on filament dynamics developed by Tabor and co-workers [2]. Wire-drag and optical tweezer approaches will be used to perturb macrofiber and to measure forces associated with motions. Models will be constructed that link the geometry of individual cell growth to forces responsible for motions. The relationship of motions to the helix hand and twist state of the final three-dimensional fiber product will also be modeled. Undergraduates, graduate students, and postdoctoral associates from all three disciplines, biology, mathematics and physics, are involved in various aspects of this project.