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But let us now try to see what the available models lack. Surprisingly there is an impressive amount of information on chromosome properties which has been left out of these graphic representations. This is apparently due to the absence of a coherent picture that integrates the interactions between the different regions. The information that is missing mainly has to do with the function of the chromosome as a whole. Without it, it seems difficult, if not impossible, to discover the mechanism responsible for the maintenance of chromosome integrity.

The ability of Gram-negative phytopathogenic bacteria to cause disease is determined by hrp/hrc genes, which encode a type-III secretion system, and hop genes, which produce the secreted effector proteins (5). A >24-kb cluster of hrp/hop genes is necessary for Pantoea stewartii subsp. stewartii to incite Stewart’s Wilt on corn (1,3). This study describes some aspects of the genetic organization and function of the regulatory region in the hrp/hop cluster of P. stewartii involved in sensing environmental signals.