choleraecells which have lost chrII. DNA degradation that accompanies the death of these cells. Overall, our findings suggest that the ParE toxins lead to the postsegregational killing of cells missing chromosome II in a manner that closely mimics postsegregational killing mediated by plasmid-encoded Cephapirin Benzathine homologs. Thus, theparDEloci aid Cephapirin Benzathine in the maintenance of the integrity of theV. choleraesuperintegron and in ensuring the inheritance of chromosome II. Toxin-antitoxin (TA) systems were first explained in low-copy-number plasmids such as F, R1, and RK2, where they promote plasmid maintenance in bacterial populations by killing plasmid-free child cells (17,34,40). This mechanism of plasmid maintenance, often referred to as postsegregational killing, Cephapirin Benzathine relies on the greater stability of the toxin than its antidote, the antitoxin. Child cells lacking a plasmid bearing a TA system cannot synthesize either TA component and thus become subject to the growth-inhibitory/killing activity of the more long-lived toxin (examined in reference16). To date, nine families of TA systems have been described (58), all of which employ Rabbit Polyclonal to UBTD1 toxins that inhibit either protein or DNA synthesis (58). TA proteins are typically encoded by an operon in which the gene encoding the antitoxin is located upstream of the gene encoding the toxin (examined in reference16). Notably, TA systems are not only encoded by low-copy-number plasmids. Nearly all completely sequenced bacterial genomes have been found to contain chromosomal TA loci (43), and some bacterial genomes harbor very large numbers of such Cephapirin Benzathine loci. For example,Mycobacterium tuberculosishas more than 80 putative TA loci (46) andEscherichia colihas at least 15 (43,44,46). Chromosomal TA loci are often clustered together and associated with mobile genetic elements (43), and paralogous loci are frequently present (43). The physiologic functions of chromosomal TA loci are not well comprehended and remain the subject of some controversy. TA systems may primarily be selfish entities that merely promote their own vertical inheritance. However, several unique biological functions for chromosomal TA systems have been proposed. There is evidence that these Cephapirin Benzathine ubiquitous loci may mediate stress responses (3,43), programmed cell death (10,26-28), persister cell generation (25,30), stabilization of large dispensable chromosome regions (55,60), and multicellular development (36). Such effects are not necessarily mutually unique. We have been investigating the activities and functions of TA loci inVibrio cholerae, the Gram-negative rod that causes cholera (2). TheV. choleraegenome, like that of all vibrios, is usually divided between two chromosomes (20). The largeV. choleraechromosome, chromosome I (chrI), is usually 2.96 Mbp and encodes nearly all of the organism’s essential genes. The smallV. choleraechromosome (chrII, 1.07 Mbp) contains a relative excess of hypothetical and uncharacterized open reading frames (ORFs), but the presence of at least a few essential genes on this replicon qualifies it as a bona fide chromosome (9). Interestingly, all 13 annotatedV. choleraeTA loci are found on chrII within a 126-kb superintegron (3,43) (observe Fig.1A). These 13 TA loci include threeparDEhomologs, as well as sevenrelBEloci, onephd/doclocus, and twohigBAloci. OnehigBA locus was shown to encode a functional TA pair whose toxin, HigB, inhibits translation by cleaving mRNA (2,4). We recently found that theparDE2locus also encodes a functional TA system (62). This locus exhibits modest similarity to the plasmid RK2parDElocus, which has been shown to inhibit DNA gyrase (23). We found that the ParE2 toxin ofV. choleraeinactivates DNA gyrase through a mechanism that differs from that of other characterized gyrase inhibitors, including the F plasmid-encoded CcdB toxin and the quinolone antibiotics (62). The ParD2 antitoxin created a protein complex with ParE2, thereby preventing its activity; however, ParD2 could not reverse ParE2 toxicity. == FIG. 1. == Autoregulation of theV. cholerae parDEloci. (A).
choleraecells which have lost chrII