Recombinational Regulation of Gene Expression in Bacteria

Abstract

INTRODUCTION
It is generally assumed that in a population derived from a single individual, e.g., a bacterial colony, the members of the population are almost all genetically identical. However, on closer inspection this assumption is found to be incorrect. Thus, e.g., Anderson and Roth (1977) have reviewed the evidence indicating that tandem duplication and amplification of regions of the Escherichia coli and Salmonella genomes arise at relatively high frequency (10⁻³/cell/generation). Therefore, in a clonal population derived after 20–30 generations of growth under selective conditions, individual cells may differ with respect to the number of copies of specific genes. Other more apparent heterogeneity is observed when bacteria are isolated and cloned from natural sources. The colonies are frequently found to be polymorphic with respect to a specific characteristic. One manifestation of this polymorphism is seen as sectored colonies. For example, when marine bacteria such as Vibrio harveyi are isolated from seawater and streaked on nutrient plates, the colonies, when observed in the dark, show sectors that are bioluminescent (see Fig. 1). If a small portion of the bioluminescent sector is picked and restreaked, each of the individual colonies is still found to be sectored, with part of the colony being bioluminescent and the other part dark (Keynan and Hastings 1961). Thus, these organisms show alternation of expression of bioluminescence, and the colony is said to be polymorphic with respect to bioluminescence.

Table 1 is a partial list of some observations of clonal dimorphism and polymorphism that have been reported. Some...