Conditionally transposition-defective derivative of Mu d1(Amp Lac)

Osmoregulation of gene expression in Salmonella typhimurium: proU encodes an osmotically induced betaine transport system

Previous evidence has indicated that a gene, proU, is involved in the response of bacterial cells to growth at high osmolarity. Using Mu-mediated lacZ operon fusions we found that transcription of the proU gene of Salmonella typhimurium is stimulated over 100-fold in response to increases in external osmolarity. Our evidence suggests that changes in turgor pressure are responsible for these alterations in gene expression. Expression of proU is independent of the ompR gene, known to be involved in osmoregulation of porin expression. Thus, there must be at least two distinct mechanisms by which external osmolarity can influence gene expression. We show that there are relatively few genes in the cell which are under such osmotic control. The proU gene is shown to encode a high-affinity transport system (Km = 1.3 microM) for the osmoprotectant betaine, which is accumulated to high concentrations in response to osmotic stress. Even when fully induced, this transport system is only able to function in medium of high osmolarity. Thus, betaine transport is regulated by osmotic pressure at two levels: the induction of expression and by modulation of activity of the transport proteins. We have previously shown that the proP gene encodes a lower-affinity betaine transport system (J. Cairney, I. R. Booth, and C. F. Higgins, J. Bacteriol., 164:1218-1223, 1985). In proP proU strains, no saturable betaine uptake could be detected although there was a low-level nonsaturable component at high substrate concentrations. Thus, S. typhimurium has two genetically distinct pathways for betaine uptake, a constitutive low-affinity system (proP) and an osmotically induced high-affinity system (proU).

Identification of a cis-acting regulatory region in the pncB locus of Salmonella typhimurium

The pncB locus of the pyridine nucleotide cycle of NAD biosynthesis in Salmonella typhimurium was examined in terms of genetic structure and regulation. The gene appears to be regulated at the level of transcription in response to the end product of the pathway, NAD. Insertions into promotor proximal regions of the gene relinquish it from regulation, while insertions into more distal regions allow retention of regulation. Regulation cannot be restored in trans to strains containing promotor proximal insertions implicating the existence of a cis acting regulatory region.

Directed formation of deletions and duplications using Mud(Ap, lac)

A genetic procedure is directed for the isolation of chromosomal deletions and duplications with predetermined endpoints. These rearrangements are generated in transduction crosses using a mixture of P22-transducing phage lysates grown on two strains, each carrying a Mud-lac insertion. The formation of duplications and deletions was demonstrated in the his operon using insertions of Mud 1-8 (a transposition-defective Mud-lac phage). This technique was also used to make larger chromosomal duplications between Mud 1-8 insertions in the thr and leu biosynthetic operons and between Mud insertions in the thr and pyrB operons. Genetic evidence is presented that strongly suggests that inheritance of a single Mud prophage by P22-mediated crosses requires two transduced fragments; each carrying part of the Mud prophage. The two fragments must be involved in three recombinational exchanges; one exchange joins the donor Mud fragments and two exchanges occur between the composite fragment and the recipient chromosome, one on either side of the complete donor Mud element. Since duplications only occur between Mud insertions in the same orientation on the chromosome, the method of duplication formation provides a simple means of determining the orientation of Mud 1-8 on the chromosome and, therefore, the direction of transcription of the gene into which Mud is inserted. This method was also used to construct recombinants between a Mud 1-8 prophage and Casadaban's protein fusion vector Mud2 and, thereby, isolate Mud2-8, a Mud derivative containing the protein fusion ability of Mud2 and the defective transposition functions of Mud1-8.