Prokaryotic Model of Epigenetic Inactivation: Chromosomal Silencing in Bacillus subtilis Fusion Products

Abstract

The study of “DNA silencing” in bacteria by an imprinting-like process is a new but expanding field (Hotchkiss and Gabor 1985; Casadesús and Maldonado 1990; Gratia 1994). To this day, only a few examples of control of gene expression through epigenetic inactivation have been reported in bacteria, unlike eukaryotic systems, where the phenomenon has been reported since 1930 (Muller 1930; Lyon 1993). Silencing involving large DNA fragments has been observed in Escherichia coli. For instance, the F plasmid encodes two proteins, SopA and SopB, and a cis-acting sopC element, which belongs to the partition system. This system is necessary for plasmid inheritance at cell division (Hiraga 1992). Recently, an additional function was demonstrated for the SopB protein, involved in DNA silencing. The SopB product acting at the sopC element in vivo promotes the inactivation of genes located within a 10-kb region adjacent to sopC. Silencing in this case is associated with the propagation of a nucleoprotein structure that commences at the sopC region. This structure restrains the accessibility of the intracellular DNA to proteins such as DNA gyrase or DNA methylase (Lynch and Wang 1995).

Another example is the E. coli bgl operon responsible for the utilization of β-glucosides. This operon is normally repressed in wild-type cells. Its silencing might be associated with promoter inactivation through a specific structural chromatin domain. It was demonstrated that promoter inactivation in this case involved the action of both upstream and downstream chromosomal elements (Schnetz 1995). Bacteria can also display heritable genetic inactivation affecting...