The IS10 antisense RNA blocks ribosome binding at the transposase translation initiation site

Mechanism of post-segregational killing by the hok/sok system of plasmid R1

The hok/sok locus of plasmid R1, which mediates plasmid stabilization by killing of plasmid-free segregants, codes for two RNAs, Hok mRNA and Sok antisense RNA. Hok mRNA encodes the Hok killer protein of 52 amino acid residues. Expression of hok is regulated post-transcriptionally by Sok antisense RNA. Killing of plasmid-free daughter-cells by the hok/sok system is accomplished through differential decay of the Hok and Sok-RNAs: Hok mRNA is very stable while Sok-RNA decays rapidly, thus leading to derepression of Hok mRNA translation in plasmid-free segregants, ensuring a rapid and selective killing of these cells. Sok antisense RNA is complementary to the leader region of the Hok mRNA. However, the region of complementarity does not overlap with the hok Shine-Dalgarno sequence. Thus, Sok-RNA regulates hok translation indirectly by an as yet unknown mechanism. We show here that Sok antisense RNA regulates the translation of another reading frame located in the hok/sok locus. This new reading frame, which overlaps with almost the entire hok gene, was denoted mok (mediation of killing). Point-mutations that prevent mok translation through the hok translational initiation region abolish efficient expression of hok. Furthermore, these mutations abolish the Sok-RNA-mediated control of hok gene expression. Hence, the antisense-RNA-mediated regulation of the hok gene seems to occur via translational coupling between the hok and mok reading-frames.

Repression and derepression of conjugation of plasmid R1 by wild-type and mutated finP antisense RNA

The finP gene of plasmid R1 is located between the genes traM and traJ, partially overlapping the first few nucleotides of the latter. It codes for a repressor of the conjugation system. The product of this gene is a small RNA of 72 nucleotides and, because it is transcribed from the opposite DNA strand, it is complementary to the 5' non-translated sequences, the ribosome-binding site, and the first two codons of traJ mRNA. The finP transcript is present in much higher concentrations in R1 than in R1-19 containing cells, the latter being a derepressed mutant of the former. A synthetic finP gene expressed from a synthetic lambda PL promoter markedly reduced the conjugation frequency of pDB12, a multicopy derivative of R1-19. Mutagenesis of finP showed that only finP loop II mutants have lost the ability to repress conjugation of R1-19 in trans. They are also the only ones which derepress conjugal DNA transfer of R1, probably by competing for the finO product, a molecule needed as corepressor for maximal activity. Mutations interrupting potential open reading frames of finP do not abolish finP repressor activity. Hence finP acts as an antisense RNA blocking the expression of the traJ gene by interacting with traJ mRNA through loop II.

The c4 repressors of bacteriophages P1 and P7 are antisense RNAs

The c4 repressors of P1 and P7 inhibit antirepressor synthesis and are solely responsible for heteroimmunity of the phages. We show that c4 is a new type of antisense RNA acting on a target, ant mRNA, that is transcribed from the same promoter. Interaction depends on complementarity of two pairs of short sequences encompassing the ribosome binding site involved in ant expression. We demonstrate that heteroimmunity of P1 and P7 is due to just two substitutions in each of the complementary sequences of c4 and ant mRNA. Based on P1-P7 sequence comparison and a mutant analysis, we propose a secondary structure model for c4 RNA, with the complementary regions in loops as important sites for antisense control.

Vectors for constructing kan gene fusions: direct selection of mutations affecting IS10 gene expression

We describe several vectors for constructing translational fusions to the kan gene of Tn5. Fusions are constructed in vitro using multi-copy vectors containing unique cloning sites situated between upstream transcriptional terminators and a downstream kan gene lacking transcriptional and translational start signals. Multi-copy fusions can be converted to single-copy chromosomal fusions by in vivo recombination with specific phage lambda vectors and vice versa. We find that kan fusions are often more suitable than lacZ fusions for the direct selection of mutations that increase fusion expression. These vectors were developed for isolating mutations that increase IS10 transposase expression; we describe strategies used to isolate such mutations, which map to IS10 or the Escherichia coli himA, himD(hip), dam or infC genes.