A control system which causes alternating turn-off/turn-on of transcription on insertion element IS1

Activity of Lac repressor anchored to the Escherichia coli inner membrane

The transient inactivation of gene regulatory proteins by their sequestration to the cytoplasmic membrane in response to cognate signals is an increasingly recognized mechanism of gene regulation in bacteria. It remained to be shown, however, whether tethering to the membrane per se could be responsible for inactivation, i.e. whether such relocation leads to a spatial separation from the chromosome that results in inactivity or whether other mechanisms are involved. We, therefore, investigated the activity of Lac repressor artificially attached to the Escherichia coli cytoplasmic membrane. We demonstrate that this chimeric protein perfectly represses transcription initiated at the tac operator–promoter present on a plasmid and even in the chromosome. Moreover, this repression is inducible as normal. The data suggest that proteins localized to the inner face of the cytoplasmic membrane in principle have unrestricted access to the chromosome. Thus sequestration to the membrane in terms of physical separation from the chromosome cannot account alone for the inactivation of regulatory proteins. Other mechanisms, like induction of a conformational change or masking of binding domains are required additionally.

High-level ribosomal frameshifting directs the synthesis of IS150 gene products

IS150 contains two tandem, out-of-phase, overlapping genes, ins150A and ins150B, which are controlled by the same promoter. These genes encode proteins of 19 and 31 kD, respectively. A third protein of 49 kD is a transframe gene product consisting of domains encoded by both genes. Specific -1 ribosomal frameshifting is responsible for the synthesis of the large protein. Expression of ins150B also involves frameshifting. The IS150 frameshifting signals operate with a remarkably high efficiency, causing about one third of the ribosomes to switch frame. All of the signals required for this process are encoded in a 83-bp segment of the element. The heptanucleotide A AAA AAG and a potential stem-loop-forming sequence mark the frameshifting site. Similar sequence elements are found in -1 frameshifting regions of bacterial and retroviral genes. A mutation within the stem-loop sequence reduces the rate of frameshifting by about 80%. Artificial transposons carrying this mutation transpose at a normal frequency, but form cointegrates at a approximately 100-fold reduced rate.

Second-element turn-on of gene expression in an IS1 insertion mutant

To learn more about the ways in which genes silenced by insertion mutations can be reactivated, we have undertaken a systematic investigation of Gal+ revertants of the polar mutant galOP-306::IS1 in Escherichia coli K12. The selective conditions used excluded reversion to wild type by precise excision of IS1. In this system (which resided on a multi-copy plasmid) reversion to the Gal+ phenotype occurred with a frequency of about 10(-7) per cell and per generation. Analysis of the revertants revealed that - with the single exception of the previously published chromosomal mutant sis1 - alterations in the structure of IS1 lead to reactivation of gal operon expression. These events fall into four classes: (I) insertion of IS2 at position 327 in IS1, insertion of IS2 at position 687 in IS1, (III) insertion of a hitherto undetected mobile element, IS150, at position 387, (IV) a 16-bp deletion encompassing IS1 coordinates 553-568. Of some 200 independent reversion events studied, all but one were of types I-III i.e. they involved the intervention of a second mobile element.

Control of phoR-dependent bacterial alkaline phosphatase clonal variation by the phoM region

phoR mutants with the wild-type Escherichia coli K-12 Var+ phoM region showed clonal variation of bacterial alkaline phosphatase synthesis, whereas mutants with the pho-510 Var- allele did not. The pho-510 mutation is responsible for the phoR mutant constitutive phenotype and probably arose in E. coli K-12 58F+ after X-ray mutagenesis over 40 years ago. I propose that the phoM region controls a change in state of bacterial alkaline phosphatase synthesis, at least in phoR mutants. Four possible molecular mechanisms for how phoM may act are discussed.