The conjugative plasmid pTR2030 encodes two bacteriophage defense mechanisms in lactococci, restriction modification (R+/M+) and abortive infection (Hsp+)

Abstract

pTR2030 is a conjugative plasmid which encodes resistance to bacteriophage in lactococci by a mechanism that aborts the phage infection (Hsp+). Subcloning and in vivo deletion events showed that two independent mechanisms of resistance are located on a 13.6-kilobase BglII fragment cloned in pSA3; one mechanism is responsible for the abortive infection, and the other encodes a restriction modification system. The introduction of pTR2030 or the recombinant plasmid pTK6 resulted in the loss of a resident restriction modification plasmid in Lactococcus lactis NCK202 which was not previously identified.

Bacteriophage and bacteriophage resistance in lactic acid bacteria

“…and all demonstrate a prefcrence for G(:-rich recognition scqucnccs, dcspitc the low GC ralios normally found in lactococcal DNA. Whilc a number of plasmids cncoding R/M activity have been localized (Table 1) and in some instances the genes have been chined [3,51,58,84], detailed molecular information has been gencratcd li)r only two systems, both identified in lactococci, Llal and ScrFl. It is intriguing thai these two systems, while giving typical R/M phenotypes, have quite unusual genetic organizations.…”

“…The conjugal plasmid pTR2030 encodes an R/M system, designated Llal, which has been subckmed and is expressed on the recombinant plasmid pTK6 [3]. This recombinant plasmid also contains the abortive infection gene ab/A (see below).…”

“…In some instances insensitive bacteria have been used in curing and conjugation studies to reveal the presence of multiple defense systems. For example, the prototype insensitive strain Lactococcus lactis subsp, lactis ME2 has been dissected to reveal at least five independent resistance systems [1][2][3][4][5]. However, it is eminently possible that the most potent defence systems, particularly if chromosomally en-coded, remain undetected.…”

Progress and potential in the biotechnology of lactic acid bacteria

“…Multiple restriction systems do however confer additive protection and their combination with an abortive infection system is an effective phage defence strategy. Indeed it has commonly been observed that naturally occurring phage resistant strains carry both systems sometimes encoded by the same plasmid [89][90][91][92]. This combination of distinct defence mechanisms is effective because the restriction system limits the number of successful infections ensuring good cell survival whereas the abortive mechanism prevent the proliferation of modified progeny phage by killing the cell prior to completion of the infective cycle [93].…”

Applications for biotechnology: present and future improvements in lactic acid bacteria

“…Clearly, a number of genetic approaches now exist for the construction of phage-resistant cultures for commercial purposes. Cloning of the genes responsible for the various phage-resistance mechanisms is in progress [75,76] and should facilitate their analyses at the molecular level with respect to expression and regulation. This information, coupled with a better understanding of the phage infection process in lactococci, should aid in the development of phage-resistant strains.…”