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Journal of Bacteriology, October 2009, p. 6436-6446, Vol. 191, No. 20
0021-9193/09/$08.00+0     doi:10.1128/JB.00864-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Comparative Biology of Two Natural Variants of the IncQ-2 Family Plasmids, pRAS3.1 and pRAS3.2 {triangledown}

Wesley Loftie-Eaton and Douglas E. Rawlings*

Department of Microbiology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

Received 1 July 2009/ Accepted 6 August 2009

Plasmids pRAS3.1 and pRAS3.2 are two closely related, natural variants of the IncQ-2 plasmid family that have identical plasmid backbones except for two differences. Plasmid pRAS3.1 has five 6-bp repeat sequences in the promoter region of the mobB gene and four 22-bp iterons in its oriV region, whereas pRAS3.2 has only four 6-bp repeats and three 22-bp iterons. Plasmid pRAS3.1 was found to have a higher copy number than pRAS3.2, and we show that the extra 6-bp repeat results in an increase in mobB and downstream mobA/repB expression. Placement of repB (primase) behind an arabinose-inducible promoter in trans resulted in an increase in repB expression and an approximately twofold increase in the copy number of plasmids with identical numbers of 22-bp iterons. The pRAS3 plasmids were shown to have a previously unrecognized toxin-antitoxin plasmid stability module within their replicons. The ability of the pRAS3 plasmids to mobilize the oriT regions of two other plasmids of the IncQ-2 family, pTF-FC2 and pTC-F14, suggested that the mobilization proteins pRAS3 are relaxed and can mobilize oriT regions with substantially different sequences. Plasmids pRAS3.1 and pRAS3.2 were highly incompatible with plasmids pTF-FC2 and pTC-F14, and this incompatibility was removed on inactivation of an open reading frame situated downstream of the mobCDE mobilization genes rather than being due to the 22-bp oriV-associated iterons. We propose that the pRAS3 plasmids represent a third, {gamma} incompatibility group within the IncQ-2 family plasmids.

* Corresponding author. Mailing address: Department of Microbiology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. Phone: 27-21-808 5848. Fax: 27-21-808 5846. E-mail: der{at}sun.ac.za

{triangledown} Published ahead of print on 14 August 2009.

Journal of Bacteriology, October 2009, p. 6436-6446, Vol. 191, No. 20
0021-9193/09/$08.00+0     doi:10.1128/JB.00864-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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