Regulation of Cell Differentiation in Caulobacter crescentus

INTRODUCTION The ability to differentiate is a fundamental function of most, if not all, forms of life. The term differentiation connotes a change in cellular structure or function due to an altered program of gene expression. The onset of a differentiation process may reflect the apparent expression of an internal clock that functions independent of environmental changes. Differentiation may also be initiated by extracellular fluctuations, which include chemical signals sent between cells or abrupt changes in available nutrients. An overriding theme in the expression of cellular differentiation, independent of the mechanism of initiation, is the temporal coordination of large numbers of genes involved in the event. In many instances the simple differentiation event culminates in the formation of subcellular structures such as the F pili in Escherichia coli (Sambucetti et al. 1982), flagella and pili in Caulobacter (Poindexter 1964; Shapiro 1976), and endospores in Bacillus (Losick and Youngman, this volume) or altered morphologies as seen in the formation of slugs in Dictyostelium (MacWilliams and David, this volume), shmoos in yeast (Klar et al., this volume), fruiting bodies in Myxobacterium (Kaiser, this volume), or aerial mycelium in Streptomyces (Chater, this volume). A particularly cogent issue in all of these systems is whether the mechanisms that regulate when to express a given set of gene products contribute to the control of the position of these gene products within the cell or the position of the differentiated cell within the organism. Questions of how cells are able to translate genetic information into spatial...