A virB-lacZ translational fusion was constructed to monitor expression of the Agrobacterium tumefaciens virB operon. Expression of the fusion gene was dependent on the presence of pTiA6 virA, virG, and a plant factor acetosyringone. Analysis of deletion mutants, constructed by exonuclease Bal31 digestion, showed that 68 residues upstream of the virB transcription initiation site was necessary for its expression. A TT - CC substitution at positions -62 and -61 led to a 7 fold reduction in virB expression. The virB upstream region contains a tetradecameric sequence, dPuT/ATDCAATTGHAAPy (D=A, G or T; Ha, C or T), that is conserved in the non-transcribed regions of all vir genes. Alteration of the position of this sequence relative to the promoter region sequences had a drastic negative effect on virB expression.
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ACKNOWLEDGEMENTS We thank Mark Mersereau for excellent technical assistance, Bonnie Allen for typing of the manuscript, Malcolm Casadaban for a gift of pMC1403, and John Larkin for a critical reading of the manuscript. This work was supported by a grant from the National Institute of Health (GM 37555). AD is the recipient of a Junior Faculty Research Award of the American Cancer Society (JFRA-170) and a McKnight Land-Grant Professorship of the University of Minnesota.
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