SUMMARY: is a rickettsial parasite of bovine erythrocytes causing world-wide economic losses in livestock production. Despite its importance, little is known about this rickettsia at a molecular level because it has not been cultured , and there is no small-animal model. Although several genes have been cloned and sequenced, the gross genome structure of the organism has not yet been well characterized. We separated intact bovine erythrocytes from leucocytes, and determined the genome size of by use of restriction endonuclease cleavage and pulsed-field gel electrophoresis (PFGE). A value of 56 mol% G + C was obtained for this genome by spectral analysis. Undigested DNA failed to migrate under several different electrophoretic conditions, indicating a circular genome. Digestions of intact DNA were performed using restriction endonucleases , and . Complete digestion with resulted in 12 distinct bands ranging in size from 14 to 170 kbp. Total size determined by addition of . cleaved the genome from three different isolates into just three fragments, of 598, 557 and 97 kbp. Incomplete digestion produced a band measuring 1250 kbp. These results indicate that has a circular genome between 1200 and 1260 kbp, with a G + C content of 56 mol%.


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