Population Dynamics of a Lac(-) Strain of Escherichia Coli during Selection for Lactose Utilization

During selection for lactose utilization, Lac(+) revertants of FC40, a Lac(-) strain of Escherichia coli, appear at a high rate. Yet, no Lac(+) revertants appear in the absence of lactose, or in its presence if the cells have another, unfulfilled requirement for growth. This study investigates more fully the population dynamics of FC40 when incubated in the absence of a carbon source or when undergoing selection for lactose utilization. In the absence of a carbon source, the viable cell numbers do not change over 6 days. When incubated in liquid lactose medium, Lac(-) cells do not undergo any measurable increase in numbers or in turbidity for at least 2 days. When FC40 is plated on lactose minimum medium in the presence of scavenger cells, the upper limit to the amount of growth of Lac(-) cells during 5 days is one doubling, and there is no evidence for turnover (i.e., a balance between growth and death). The presence of a minority population that could form microcolonies was not detected. The implications of these results, plus the fact that the appearance of Lac(+) revertants during lactose selection is nearly constant with time, are discussed in reference to several models that have been postulated to account for adaptive mutations.     

Lactose permease mutants which transport (malto)-oligosaccharides.

Lactose permease mutants, which were previously isolated in sugar specificity studies, were screened for their abilities to transport the trisaccharide maltotriose. Six multiple mutants (e.g., five double mutants and one triple mutant) were identified as forming fermentation-positive colonies on maltotriose MacConkey plates and were also shown to grow on maltotriose minimal plates. All of these multiple mutants contained a combination of two or three amino acid substitutions at position 177, 236, 306, or 322 within the permease. In contrast, none of the corresponding single mutants at these locations were observed to exhibit an enhanced rate of maltotriose transport. In whole-cell assays, the multiple mutants were shown to transport relatively long alpha-nitrophenylglucoside (alpha NPG) molecules. In certain cases, alpha NPG molecules containing up to four glucose residues in addition to the nitrophenyl group were shown to be transported to a significant degree. Overall, the abilities of lactose permease mutants to transport maltotriose and long alpha NPGs are discussed with regard to the dimensions of the sugar and the mechanism of sugar transport.     

Genetic Map of the Lactose Repressor Gene (i) of ESCHERICHIA COLI

39 tonB i^- deletions were used to map 57 independently isolated i gene mutants. Methods selective for i⁺ recombinants from various types of i mutants are described. i⁺ recombination frequencies of 6 x 10^-6 to 6 x 10^-7 can be detected in the different selective systems. Twenty i^s mutations and 12 i^-d mutations map in distinct but different regions of the gene. i^-_sus mutations are scattered over the gene.     

Expression of a Lactose Transposon (Tn951) in Zymomonas mobilis

The potential utility of Zymomonas mobilis as an organism for the commercial production of ethanol would be greatly enhanced by the addition of foreign genes which expand its range of fermentable substrates. We tested various plasmids and mobilizing factors for their ability to act as vectors and introduce foreign genes into Z. mobilis CP4. Plasmid pGC91.14, a derivative of RP1, was found to be transferred from Escherichia coli to Z. mobilis at a higher frequency than previously reported for any other plasmids. Both tetracycline resistance and the lactose operon from this plasmid were expressed in Z. mobilis CP4. Plasmid pGC91.14 was stably maintained in Z. mobilis at 30°C but rapidly lost at 37°C.     

Lactose permease as a paradigm for membrane transport proteins (Review)

Our structural knowledge of the major facilitator superfamily (MFS) has dramatically increased in the past year with three structures of proteins from the MFS (oxalate/formate antiporter; lactose/proton symporter and the P(i)/glycerol-3-phosphate antiporter). All three structures revealed 12 transmembrane helices forming two distinct domains and could imply that members of the MFS have preserved both secondary as well as tertiary structural elements during evolution. Lactose permease, a particularly well-studied member of the MFS, has been extensively explored by a number of molecular biological, biochemical and biophysical approaches. In this review, we take a closer look at the structure of LacY and incorporate a wealth of biochemical and biophysical data in order to propose a possible mechanism for lactose/proton symport. In addition, we make some brief comparisons between the structures of LacY and GlpT.     

Lactose permease as a paradigm for membrane transport proteins (Review)

Our structural knowledge of the major facilitator superfamily (MFS) has dramatically increased in the past year with three structures of proteins from the MFS (oxalate/formate antiporter; lactose/proton symporter and the P_i/glycerol-3-phosphate antiporter). All three structures revealed 12 transmembrane helices forming two distinct domains and could imply that members of the MFS have preserved both secondary as well as tertiary structural elements during evolution. Lactose permease, a particularly well-studied member of the MFS, has been extensively explored by a number of molecular biological, biochemical and biophysical approaches. In this review, we take a closer look at the structure of LacY and incorporate a wealth of biochemical and biophysical data in order to propose a possible mechanism for lactose/proton symport. In addition, we make some brief comparisons between the structures of LacY and GlpT.     

赤峰锦蛇ELAPHE ANOMALA为一有效种（蛇目）

本物种为Ｂｏｕｌｅｎｇｅｒ１９１６年第１次报道,标本采自中国赤峰,长期以来一直被作为棕黑锦蛇的一个亚种。作者经过三十余年的调查和饲养研究,发现其形态特征和地理分布均与棕黑锦蛇Ｅｌａｐｈｅ ｓｃｈｒｅｎｃｋｉｉ（Ｓｔｒａｕｃｈ）有明显的不同。因此,确认其为一独立有效种。     

Lactose biosensor based on Langmuir-Blodgett films of poly(3-hexyl thiophene)

An amperometric lactose biosensor was developed by immobilizing lactase (EC 3.2.1.23) and galactose oxidase (GaO) (EC 1.1.3.9) in Langmuir-Blodgett (LB) films of poly(3-hexyl thiophene) (P3HT)/stearic acid (SA) for estimation of lactose in milk and its products to prevent "lactose intolerance". The enzyme immobilized LB film was used as working electrode and platinum as reference electrode. The enzyme electrodes show a linearity 1-6 g/dL of lactose and have a shelf life more than 120 days. The reusability of electrode was found ten times with 3% loss in current response. The enzyme electrode was characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and kinetic parameters such as pH, temperature and stability. The working electrode may be used for the estimation of lactose/galactose in food and biological fluids.     

Formation of the tandem repeat (IS30)2 and its role in IS30-mediated transpositional DNA rearrangements

Plasmids carrying two IS30 elements in the same orientation, as in the composite transposon Tn2706, are structurally unstable in Escherichia coli. A primary segregation product is formed by site-specific deletion of the sequences carried between the two IS30 elements. The resulting covalently closed replicon carries the two IS30 elements as tandem repeats separated by only 2 bp. This (IS30)₂ structure is extremely unstable, but it can nevertheless be isolated on its vector plasmid and, after purification, can be reintroduced into host cells by transformation. Among the descendants of transformants of recA ^? bacteria, replicated copies of the introduced (IS30)₂ structure are still present, together with various kinds of segregation products which provide evidence for the efficient generation of DNA rearrangements. Most abundant is the product of another site-specific recombination between two identical ends of the IS30 elements involved, which results in the presence of just one intact IS30 on the plasmid. Apart from this, and depending on the presence of appropriate targets for IS30 transposition, various transposition products of (IS30)₂ are also seen. Intramolecular reactions lead to DNA inversions and deletions with breakpoints other than IS30 ends. In intermolecular reactions inverse transposition occurs at high frequency and one also obtains simple transposition and cointegration. A mutational study revealed the requirement in cis of one intact IS30 transposase gene and of both proximal ends of the two IS30 elements concerned not only for the formation of (IS30)₂, but also for its further rearrangement reactions, including the efficient formation of site-specific deletions. A model is proposed, which postulates that (IS30)₂ intermediates play a key role in IS30 transposition pathways in which the formation of (IS30)₂ may be rate-limiting. Once this structure is formed, it gives rise to a burst of transpositional rearrangements in the subclone carrying (IS30)₂. Evolutionary implications of these findings are discussed.     

Lactose intolerance in captive nocturnal prosimians (Perodicticus potto): A twenty-one year record

A colony of pottos was captured in December 1959 and brought to the United States. At that time part of their diet consisted of a high protein porridge made with whole milk. In addition, their drinking water contained an antibiotic to protect them from possible infection resulting from the change in habitat. No intestinal incontinence resulted from this treatment. Antibiotic addition to the diet ceased in 1961. In 1971 the remaining pair of animals developed a calcium deficiency. This was alleviated by adding 5 g calcium lactate to the drinking water and serving whole milk every other day. In 1977 the male developed acute lactose intolerance. His feces became bacteriologically sterile. His mate, who died on January 9, 1979, produced feces containing enteric organisms throughout her life. A large number of dairy products were served the animals in the attempt to alleviate this problem. Acidophilus milk or tinned 2% milkfat containing milk with 100% of the lactose hydrolyzed, produced no intestinal incontinence in the animal. It is suggested that sterile stools resulted from a fungal infection that killed theEscherichia coli. In addition it is proposed that non-pathogenic bacteria brought about an alteration of the brush border of the columnar epithelial cells of the villi which synthesize the enzyme lactase, such that lactase production was seriously reduced.     

Transposition of IS1-lambdaBIO-IS1 from a bacteriophage lambda derivative carrying the IS1-cat-IS1 transposon (Tn9)

Summary Tn9 is a transposable element in which a gene (cat) determining chloramphenicol resistance is flanked by directly repeated sequences that are homologous to the insertion sequence IS1. We show here that infection of Escherichia coli K12 (under Rec^- Red^- Int^- conditions) with a bio transducing phage carrying Tn9 results in the formation of bio transductants as frequently as cat transductants (about 1 per 10⁶ to 10⁷ infected cells). Most of the bio transductants do not carry cat, just as most of the cat transductants do not carry bio. In spite of the absence of cat, the bio prophage can transpose a second time, from the E. coli chromosome to different sites on an F gal plasmid. Analysis of the structure of the transposed bio element, by restriction nuclease digestion and by electron microscopy, demonstrates that the integrated bio prophage is flanked by directly repeated IS1 elements. We conclude that there is no genetic information for the ability to transpose encoded in the non-repeated portion of Tn9, i.e. that the directly repeated IS1 elements alone are responsible for Tn9 transposition.