Mobilization protein-DNA binding and divergent transcription at the transfer origin of the Thiobacillus ferrooxidans pTF1 plasmid.

The possible interaction of the trans-acting mobilization proteins, MobL and MobS, at the cognate origin of transfer (oriT) region of the Thiobacillus ferrooxidans plasmid pTF1 has been investigated. In gel retardation assays with crude protein extracts from overproducing strains, a truncated MobL (c. 28 kDa) as well as its native protein (42 kDa), but not the 11 kDa MobS protein, were found to bind specifically to a 42-mer oligonucleotide which represents the transferred DNA strand of the minimal oriT fragment of pTF1. In vivo, the binding of MobL was studied by monitoring catechol 2,3-dioxygenase (xylE) activities driven by promoters of the divergently transcribed mobL and mobS genes. The mob promoter sequences were found to resemble the Escherichia coli sigma 70-dependent consensus promoter elements. The '-10' recognition sequences of mobL and one of the two mobS promoters overlap except for one base and they are positioned within the putative 'hairpin' structure in the minimal oriT sequence. In accordance with the twin supercoil-domain model of Liu and Wang (1987) which suggests that transcription can generate local variations in DNA superhelicity, we propose a possible physiological role of DNA supercoiling in the transfer origin with reference to divergent transcription of mobL and mobS genes.