Disco-ordinate expression of the Escherichia coli gal operon after prophage lambda induction

Further inducibility of a constitutive system: ultrainduction of the gal operon

In wild-type Escherichia coli, expression of the gal operon is negatively regulated by the Gal repressor and is induced 10- to 15-fold when the repressor is inactivated by an inducer. In strains completely deleted for galR, the gene which encodes the Gal repressor, the operon is derepressed by only 10-fold without an inducer. But this derepression is increased further by threefold during cell growth in the presence of an inducer, D-galactose or D-fucose. This phenomenon of extreme induction in the absence of Gal repressor is termed ultrainduction--a manifestation of further inducibility in a constitutive setup. Construction and characterization of gene and operon fusion strains between galE and lacZ, encoding beta-galactosidase as a reporter gene, show that ultrainduction occurs at the level of transcription and not translation. Transcription of the operon, from both the cyclic AMP-dependent P1 and the cyclic nucleotide-independent P2 promoters, is subject to ultrainduction. The wild-type galR+ gene has an epistatic effect on ultrainducibility: ultrainduction is observed only in cells devoid of Gal repressor protein. Titration experiments show the existence of an ultrainducibility factor that acts like a repressor and functions by binding to DNA segments (operators) to which Gal repressor also binds to repress the operon.

Analysis of relative reversion frequencies for IS2 insertion mutations in the regulatory region of the galOPETK operon of Escherichia coli

Two previously characterized mutations in the galOPETK operon of Escherichia coli, galOP-3 and galOPE-490, contain IS2 insertions only 1 bp apart in the gal regulatory region; yet only the former yields Gal+ phenotypic revertants at a detectable frequency. We have shown that the galOPE-490 allele comprises two mutations--an IS2(I) insertion at bp+(2-6) (relative to the gal mRNA start site) plus a C/G to A/T transversion at bp + 59. The latter creates an ochre stop codon and lies within the internal site of the bipartite gal operator; it acts as an operator mutation in an in vivo repressor titration assay. Analysis of a newly isolated allele (galOP-490*) which retains the IS2 of galOPE-490 but is galE+ reveals a reversion frequency approximately 30-fold higher than that of galOP-3. Reversion of galOPE-490 is at least 10,000-fold lower and has not been detectable even under conditions conducive to enhanced double mutations in other systems.

Construction and properties of plasmid pKC30, a pBR322 derivative containing the pL-N region of phage lambda

The phage lambda N gene, contained on a HindIII-BamHI lambda DNA fragment (lambda coordinates 38 695-34 500), was inserted into plasmid pBR322 and cloned in Escherichia coli containing a defective lambda prophage. The plasmid is called pKC30. lambda N-mediated pL transcription interfered with plasmid pKC30 maintenance. Plasmid pKC30 was stabilized (i) by repression of pL, (ii) by a nutL mutation, or (iii) by cloning an "N-unresponsive terminator" sequence downstream from N gene.

Molecular cloning and sequence analysis of trp-lac fusion deletions

DNA fragments containing deletions that fuse the trp operon to the lac operon were cloned and the end-points of the fusions were determined. The results from DNA sequence analysis correlated well with those from genetic, biochemical and physiological studies previously reported. The sequence data from this study, in combination with the known properties of these fusion strains, provided information on: (1) the precise lac operon distal boundary of the lac operator; (2) the nature of the trp operon terminator; and (3) the messenger RNA sequences that result in inhibition of lacZ translation initiation in trp-lac fused mRNAs.

Escherichia coli gal operon proteins made after prophage lambda induction

Expression of the EScherichia coli gal operon under the control of the prophage lambda promoter pL leads to gross discoordinacy of gal expression. Expression of the most promoter-distal cistron galK is much greater than expression of the promoter-proximal cistron galE. We had previously shown that transcription of the gal operon is coordinate after prophage induction. A survey of protein synthesized after prophage induction indicated that lack of expression of galE is due to a failure of translation of the galE sequence in the pL-gal transcript. This failure of translation of the galE sequence may be due to extensive dyad symmetry present in the vicinity of the gal promoter region of the pL-gal transcript. This symmetry could result in a ribonucleic acid stem-loop structure, blocking the attachment of ribosomes at the Shine-Dalgarno sequence of galE. To test this model, strains bearing the IS1 or IS2 insertion, deletion, or new promoter mutation within the symmetrical region were constructed. The restoration of some galE expression after such disruptions of the symmetrical region indicated that the ribonucleic acid stem-loop structure did play a role in the discoordinate expression of gal from pL. However, failure to obtain galE expression coordinated with high levels of galK expression suggested that other components were involved, perhaps other symmetries between galE and the pL transcript.

The nus mutations affect transcription termination in Escherichia coli

The nusA1 and nusB5 mutations in a partial suppression of polarity and thus transcription termination in Escherichia coli. As these mutations block the transcription antitermination activity of bacteriophage lambda N gene product, they paradoxically seem to enhance transcription termination at phage termination sites. The rho mutation HDF026 displays almost identical properties. The observations suggest that the nusA and nusB gene products may act as termination factors analogous to rho protein.

Differential translation efficiency explains discoordinate expression of the galactose operon

We have used an mRNA-dependent E. coli S-30 translation system to compare the translation efficiencies of two polycistronic transcripts of the galactose operon, the CRP-cAMP-dependent mRNA (P1) and the CRP-cAMP-independent mRNA (P2). The RNAs were prepared in vitro, quantitated by hybridization or gel analysis and translated in a cell free system. The specific protein products were measured, and their quantities were compared with the amount of input mRNA. Our results show that the P2 mRNA synthesizes epimerase, the 5'-proximal gene product of the gal operon, four times more efficiently than the P1 mRNA. The 5'-distal gene products, transferase and kinase, are translated with the same efficiency from both transcripts. Thus the ratio of epimerase to kinase synthesis is four times higher for the P2 mRNA than for the P1 mRNA. This change in epimerase to kinase ratio is identical to that observed in vivo when the cellular cAMP level falls and gal transcription is believed to switch from P1 to P2. We suggest that it is the differential translation efficiency of the epimerase gene on the two different gal transcripts that accounts for this discoordinate expression. Moreover, since the P2 mRNA differs from the P1 mRNA only by the addition of five nucleotides at the 5' terminus and these nucleotides are outside the ribosome binding region we determine for epimerase, the selective difference in the translation efficiency of epimerase is probably mediated by RNA conformation.