Summary The silent bgl operon of Escherichia coli is activated by spontaneous mutations that derepress its promoter. In addition, expression depends on specific transcriptional antitermination within the operon by the antiterminator protein BglG. Here, we show that BglG‐mediated antitermination limits expression of the bgl operon when the cellular transcription rate is low. The expression levels of chromosomally encoded activated bgl operon alleles are low but increase significantly when BglG protein is provided in trans or when the expression is rendered inde‐pendent of BglG‐mediated antitermination by mutation of the terminator. Plasmid‐encoded activated bgl operon alleles are expressed at high levels. Moreover, a moderate (threefold) further increase in the transcription rate of chromosomally encoded activated bgl operon alleles in an rpoS mutant can result in high (up to 50‐fold increased) expression levels. These data show that the expression of the bgl operon does not correlate linearly with its cellular transcription rate. Moderate differences in the transcription initiation rate are amplified post‐transcriptionally into large changes in the expression level of the operon by the requirement of a threshold for BglG‐mediated antitermination. Implications for bgl operon regulation by global regulators H‐NS, RpoS and others are discussed.

The maize transposable element Activator (Ac) encodes a transposase (TPase) protein, whose DNAbinding domain is located in a basic region around aa 200. The N-terminal 102 aa of the TPase are not required for the transposition reaction. In transfected petunia protoplasts, we analyzed the protein levels of the N-terminafly truncated TPase and mutants thereof and the corresponding transposition frequencies. The TPase protein forms large insoluble aggregates at high expression levels. There is no proportionality observed between TPase levels and transposition frequency. Twenty-one mutations (of 26), which are distributed over the whole length of the protein, inactivate the TPase completely. By coexpressing inactive mutant and active truncated TPase, it was found that several mutations have a trans-dominant inhibitory effect. Among those are two DNA-binding-deficient mutants, indicating that inhibition of the active TPase is not caused by competition for the binding sites on the transposon. Accordingly, Ac TPase acts as an oligo-or multimer formed by protein-protein interactions. Peculiarly, two mutants lacking 53 and 98 aa from the C terminus that are themselves transpositionally inactive lead to an increased excision frequency when they are coexpressed with the active truncated TPase.

IS-elements are devoid of easily recognizable transacting functions and exert their visible effects in the position cis only (recent reviews Calos and Miller 1980; Starlinger 1980). It has been a matter of debate, whether these elements encode functions for their own transposition. In the case of the E. coli IS-elements this could not easily be determined by genetic methods, because most of these elements are present in several copies (Saedler and Heiss 1973; Deonier et al. 1979). In the case of the IS-elements flanking transposons, evidence has recently been brought forward that these carry the transposition specificity (Rothstein et al. 1980; Kleckner 1980; Grindley 1981). IS4 is present in one copy only in several E. coli K12 strains and should, therefore, be suitable for genetic and physiological studies (Chadwell et al. 1979). It has been cloned from several sites on the E. coli chromosome in pBR322 (Klaer and Starlinger 1980). Here we report the DNA sequence of IS4 which contains an open reading frame for 442 amino acids, and of the junctions of this element with surrounding DNA at three different sites in the E. coli chromosome.

Four insertions of IS1 in the leader sequence of the gal operon of E. coli have been analysed. Two of them occur at the same position, but in opposite orientations. The other two are inserted one nucleotide to one side and four nucleotides to the other side, respectively. In each case, nine base pairs of the leader sequence of the gal operon are duplicated directly, and are found flanking the termini of IS1 at its junction with the gal operon. These repeated sequences differ from each other as expected from the different insertion sites.

The properties have been investigated of two deletion derivatives of the transposase protein (TPase) of maize transposable element Ac in transgenic tobacco. The wild‐type and mutant TPases were expressed as fusions to the cauliflower mosaic virus 35S promoter. A deletion of 102 amino acids from the N‐terminus, TPase(103–807), induces Ds excisions from a SPT::Ds reporter locus with a higher frequency than the wild‐type TPase. The increased transpositional activity of TPase(103–807) is a dominant trait, as seedlings coexpressing truncated and wild‐type TPase show the characteristic TPase(103–807) variegation pheno‐type. A transpositionally inactive TPase deletion derivative lacking 188 amino acids from the N‐terminus inhibits the transpositional activity of the wild‐type TPase.