HOMOLOGY BETWEEN A GENETIC-LOCUS (MDOA) INVOLVED IN THE OSMOREGULATEDBIOSYNTHESIS OF PERIPLASMIC GLUCANS IN ESCHERICHIA-COLI AND A GENETIC-LOCUS (HRPM) CONTROLLING PATHOGENICITY OF PSEUDOMONAS-SYRINGAE

Membrane-derived oligosaccharides (MDO) of Escherchia coli are represe ntative members of a family of glucans found in the periplasmic space of Gram-negative bacteria. The two genes forming the mdoGH operon are necessary for the synthesis of MDO. The nucleotide sequence (4759 bp) and the transcriptional start of this operon were determined. Both gen e products were further characterized by gene fusion analysis. MdoG is a 56 kDa periplasmic protein whose function remains to be determined. MdoH, whose presence was shown to be necessary for normal glucosyl tr ansferase activity, is a 97 kDa protein spanning the cytoplasmic membr ane. To our surprise, these proteins are not homologous to the peripla smic glucan biosynthetic enzymes previously characterized in the Rhizo biaceae family. However, a considerable homology (69% identical nucleo tides out of 2816) was discovered between mdoGH and the two genes pres ent at the hrpM locus of the phytopathogenic bacterium Pseudomonas syr ingae pv. syringae. Functions of these genes remain mysterious but the y are known to be required for both the expression of disease symptoms on host plants and the development of the hypersensitive reaction on non-host plants (Mills and Mukhopadhyay, 1990). These results confirm the importance of periplasmic glucans for the physiological ecology of Gram-negative bacteria.