Moraxella bovis, a Gram-negative bacterium belonging to the family Neisseriaceae, is the aetiological agent of infectious bovine keratoconjuctivitis, a disease causing significant economic losses in the cattle industry worldwide. Potential virulence determinants produced by M. bovis include type IV pili, RTX toxin, proteases, fibrynolysines, and phospholipases. The plasmids are known to play an important role in the virulence of many bacterial species. Most isolates of M. bovis contain multiple plasmids, but as yet little information on the genes encoded by them have been reported.

The complete nucleotide sequence of the plasmid pMbo4.6 from the Moraxella bovis strain ATCC 10900 was determined. The plasmid was analysed and found to be 4658 in size with a G+C content of 38.6 mol%. Computer analysis of the sequence data revealed three major open reading frames encoding putative proteins of 64.2 (ORFI), 45.7 (ORFII) and 12.1 kDa (ORFIII). ORFI encodes a protein that shows a high level of amino acid sequence similarity (44%) with some plasmid mobilization proteins. Upstream of ORFI, we identified a 26 bp DNA fragment identical with the promoter region of the mobC gene associated with the mobilizable plasmid pEMCJH03 from Moraxella catarrhalis. ORFII encodes a protein showing a relatively high amino acid sequence similarity (about 40%) with several plasmid replication proteins. The origin of replication of many plasmids contains directly repeated sequences ranging between 19 and 22 bp each. These direct repeats are termed iterons and have been identified in several plasmids. The iteron sequences serve as binding sites for plasmid-encoded initiator proteins collectively known as Rep, for replication. Upstream of ORFII four sets of repeated sequences resembling iteron structures were identified. ORFIII encodes a protein of unknown function belonging to the DUF family and shows a similarity to ORF8 of the plasmid pMBO-1 from the Moraxella bovis Epp63 strain. Several interesting palindromic sequences, repeat sequences and hairpin-loop structures around ORFII, which might confer regulatory effects on the replication of the plasmid, were also noted. A shuttle vector capable of transforming bacteria of the Neisseriaceae family was constructed by cloning pMbo4.6 into pBR322.

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1. Molecular analysis of the maintenance mechanism of plasmid pEC156 carrying EcoVIII restriction-modification system in bacteria related to Escherichia coli.