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Applied and Environmental Microbiology, February 2001, p. 929-937, Vol. 67, No. 2
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.2.929-937.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Naturally Occurring Lactococcal Plasmid pAH90 Links Bacteriophage Resistance and Mobility Functions to a Food-Grade Selectable Marker

David O' Sullivan,1,2 R. Paul Ross,1,* Denis P. Twomey,1,2 Gerald F. Fitzgerald,2 Colin Hill,2 and Aidan Coffey1

Teagasc, Dairy Products Research Centre, Moorepark, Fermoy,1 and Microbiology Department & National Food Biotechnology Centre, University College Cork, Cork,2 Ireland

Received 22 May 2000/Accepted 23 October 2000

The bacteriophage resistance plasmid pAH90 (26,490 bp) is a natural cointegrate plasmid formed via homologous recombination between the type I restriction-modification specificity determinants (hsdS) of two smaller lactococcal plasmids, pAH33 (6,159 bp) and pAH82 (20,331 bp), giving rise to a bacteriophage-insensitive mutant following phage challenge (D. O'Sullivan, D. P. Twomey, A. Coffey, C. Hill, G. F. Fitzgerald, and R. P. Ross, Mol. Microbiol. 36:866-876; 2000). In this communication we provide evidence that the recombination event is favored by phage infection. The entire nucleotide sequence of plasmid pAH90 was determined and found to contain 24 open reading frames (ORFs) responsible for phenotypes which include restriction-modification, phage adsorption inhibition, plasmid replication, cadmium resistance, cobalt transport, and conjugative mobilization. The cadmium resistance property, encoded by the cadA gene, which has an associated regulatory gene (cadC), is of particular interest, as it facilitated the selection of pAH90 in other phage-sensitive lactococci after electroporation. In addition, we report the identification of a group II self-splicing intron bounded by two exons which have the capacity to encode a relaxase implicated in conjugation in gram-positive bacteria. The functionality of this intron was evident by demonstrating splicing in vivo. Given that pAH90 encodes potent phage defense systems which act at different stages in the phage lytic cycle, the linkage of these with a food-grade selectable marker on a replicon that can be mobilized among lactococci has significant potential for natural strain improvement for industrial dairy fermentations which are susceptible to phage inhibition.

* Corresponding author. Mailing address: Teagasc, Dairy Products Research Centre, Moorepark, Fermoy, Co. Cork, Ireland. Phone: 353-25-42229. Fax: 353-25-42340. E-mail: pross{at}moorepark.teagasc.ie.

Applied and Environmental Microbiology, February 2001, p. 929-937, Vol. 67, No. 2
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.2.929-937.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.