Control of gene expression by a mobile recombinational switch.

Abstract

Transposable recombinational switches may play important roles in the evolution of bacterial populations by increasing flexibility in the control of expression of particular genes and thereby maintaining heterogeneity in clones of cells growing in a unform environment. Experiments reported here show that Tn5-112, a deletion derivative of kanamycin-resistance transposon Tn5, can function as such a mobile recombinational switch. The internal deletion in Tn5-112 removes transcription termination signals and permits transcription initiated within the element to continue into nearby bacterial genes. Consequently, in one orientation Tn5-112 stimulates distal gene expression, whereas in the other orientation the normal polarity imposed by wild-type Tn5 intervenes and distal gene expression is not stimulated. Because Tn5-112 contains terminal inverted repeats, intramolecular recombination can invert the Tn5-112 element and alter gene expression. Tn5-112 is transposition deficient. Its mobility derives from the recessive nature of the transposition deficiency and, in this study, from the possibility of homologous recombination which permits its placement in either orientation at any site occupied by another Tn5 element. The first demonstration of transposable elements came from analyses of mutable loci in maize. Discrete genetic elements that could move from site to site were recognized as agents that induced chromosome aberrations and that modified the activity of genes near their sites of insertion in a developmentally regulated fashion. It was suggested that such elements might also control the activities of many genes during normal development (1,2).The many recently discovered bacterial transposons (3, 4) led to the intriguing evolutionary concept of genome plasticity-that genes can be gained, lost, or shifted to new positions by processes that do not require extensive nucleotide sequence homology. The potential involvement of transposons in the evolution of new regulatory systems is also apparent: These elements create polar mutations which block transcription of genes in an operon distal to their insertion sites (3). Two transposons (IS2 and Tn3) contain promoters that stimulate the expression of distal genes when the elements are inserted in the correct orientation (5, 6). One transposon (Tn5) stimulates distal gene expression from a fraction of its sites of insertion, independently of its orientation, as if new promoters are created by the fusion of Tn5 with certain target sequences (7).Many transposons contain long terminal inverted repetitions.Because a crossover between inverted repeats inverts interstitial unique sequences (Fig. 1) I report here that Tn5-112, a simple deletion derivative of the kanamycin-resistance transposon Tn5, has gained the ability to act as a recombinational switch which turns on distal gene expression from an internal promoter in one orientation but not in the other. By homologous recombination one can both place in either orientation at any site occupied by another Tn5 elem...

Recombinogenic properties of herpes simplex virus type 1 DNA sequences resident in simian virus 40 minichromosomes

“…inhibition of recombination between inverted repeat sequences in SV40 minichromosomes have been recently described (10). Thus, the observation that TnS inversion is prohibited in supercoiled covalently closed circular DNA such as the SV40 minichromosome and an Escherichia coli plasmid but not in linear DNA such as the HSV-1 genome (41), the bacteriophage A genome (43), and the E. coli chromosome (1) suggests that it may be mechanistically impossible for any recombinase, including that encoded by HSV-1, to promote inversion events in this form of DNA. The existence of such topological constraints within this SV40 shuttle vector indicates that it should not be employed in an analysis to determine whether the recombinase built into the HSV-1 DNA replication machinery can also function through other DNA replication systems.…”

Nucleotide sequence of a repetitive element isolated from Leptospira interrogans serovar hardjo type hardjo-bovis

“…A significant feature of this repetitive element is the high copy number in L. hardjo-bouis and also L. saxkoebing. It is generally assumed that saturating numbers of IS elements and transposons are potentially lethal to a cell and selfregulation mechanisms to repress transposition have evoived (Berg, 1980). It is possible that a derepressed burst of transposition saturated the chromosome with many copies of the element and gave rise to the L. hardjobocis clone.…”

Simple assay for quantitation of Tn5 inversion events in Escherichia coli and use of the assay in determination of plasmid copy number

“…However, the apparent inversion of TnS sequences has been detected by using two specialized systems. In the first, inversion of a Tn5 deletion derivative integrated in the E. coli chromosome resulted in the activation of an adjacent lacZ gene (2,4), while in the second, inversion of a Chi' derivative of Tn5 in the bacteriophage A genome led to an altered plaque morphology (18). These observations indicate that Tn5 does undergo sequence inversion in its bacterial host, albeit at a low frequency.…”