Positive selection vectors based on xylose utilization suppression

Ethanol and thermotolerance in the bioconversion of xylose by yeasts

The mechanisms underlying ethanol and heat tolerance are complex. Many different genes are involved, and the exact basis is not fully understood. The integrity of cytoplasmic and mitochondrial membranes is critical to maintain proton gradients for metabolic energy and nutrient uptake. Heat and ethanol stress adversely affect membrane integrity. These factors are particularly detrimental to xylose-fermenting yeasts because they require oxygen for biosynthesis of essential cell membrane and nucleic acid constituents, and they depend on respiration for the generation of ATP. Physiological responses to ethanol and heat shock have been studied most extensively in S. cerevisiae. However, comparative biochemical studies with other organisms suggest that similar mechanisms will be important in xylose-fermenting yeasts. The composition of a cell's membrane lipids shifts with temperature, ethanol concentration, and stage of cultivation. Levels of unsaturated fatty acids and ergosterol increase in response to temperature and ethanol stress. Inositol is involved in phospholipid biosynthesis, and it can increase ethanol tolerance when provided as a supplement. Membrane integrity determines the cell's ability to maintain proton gradients for nutrient uptake. Plasma membrane ATPase generates the proton gradient, and the biochemical characteristics of this enzyme contribute to ethanol tolerance. Organisms with higher ethanol tolerance have ATPase activities with low pH optima and high affinity for ATP. Likewise, organisms with ATPase activities that resist ethanol inhibition also function better at high ethanol concentrations. ATPase consumes a significant fraction of the total cellular ATP, and under stress conditions when membrane gradients are compromised the activity of ATPase is regulated. In xylose-fermenting yeasts, the carbon source used for growth affects both ATPase activity and ethanol tolerance. Cells can adapt to heat and ethanol stress by synthesizing trehalose and heat-shock proteins, which stabilize and repair denatured proteins. The capacity of cells to produce trehalose and induce HSPs correlate with their thermotolerance. Both heat and ethanol increase the frequency of petite mutations and kill cells. This might be attributable to membrane effects, but it could also arise from oxidative damage. Cytoplasmic and mitochondrial superoxide dismutases can destroy oxidative radicals and thereby maintain cell viability. Improved knowledge of the mechanisms underlying ethanol and thermotolerance in S. cerevisiae should enable the genetic engineering of these traits in xylose-fermenting yeasts.

Positive selection vectors

This review describes information concerning positive selection vectors on their mechanism, classification, property, and limitation. A total of 72 positive selection vectors collected were discussed. Positive selection vectors can reduce background and directly screen transformants containing cloned DNA fragments. The mechanisms to perform positive selection include insertional inactivation and the replacement of functional genes of the vectors. In general, the former is much more convenient than the latter. The functional genes are controlled either by their promoters or by heterologous promoters introduced. On the basis of the structures, positive selection vectors could be classified into five groups. The positive selection vectors are commonly based on the mechanisms of lethal genes and the sensitivity of compounds. The vectors, with molecular weights ranging from 2.6 to 17.0 kb, have diverse genetic markers and wide host ranges, including Escherichia coli, Bacillus, Streptomyces, lactic acid bacteria, yeasts, and mammalian cells. Although some limitations exist for using some positive selection vectors, they are useful in recombinant DNA experiments.

Promoter analysis and transcriptional regulation of Lactobacillus pentosus genes involved in xylose catabolism

The xyl genes in Lactobacillus pentosus are induced by xylose and repressed by glucose, ribose, and arabinose. Northern blot analysis showed that regulation is mediated at the transcriptional level. Under inducing conditions, two xylA transcripts were detected, a major transcript of 1.5 kb and a minor transcript of 3 kb. The 3 kb transcript also comprises sequences from xylB, suggesting that xylA and xylB are transcribed together. A 1.2 kb xylR transcript was found under inducing and non-inducing conditions. In the presence of xylose, a second xylR transcript (> 7 kb) was detected, which includes sequences from two upstream genes, xylQ and xylP. The transcription start sites for xylA and xylR were mapped by primer extension and S1 nuclease experiments at 42 and 83 nucleotides, respectively upstream of the translation start sites. Induction by xylose of the chloramphenicol acetyltransferase (CAT) gene under control of the xylA promoter, on a multicopy plasmid, was 60 to 80-fold, but only 3 to 10-fold in the presence of glucose and xylose. Expression of CAT under control of the xylR promoter was constitutive at a level tenfold less than that observed under control of the xylA promoter. Sequence analysis suggests the presence of two operator-like elements, one overlapping with the promoter -35 region of xylA and controlling the expression of xylA by binding factors involved in catabolite repression, and a second operator downstream of the promoter -10 region of xylA, which may bind the product of xylR, the repressor.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct selection shuttle plasmid vector, pPW264, used for cloning the alpha-amylase gene of Schwanniomyces occidentalis

We constructed a novel cloning system with positive selection for inserted fragments. The gene for tetracycline resistance (tetR) originally used in plasmid pTR262 was replaced with the gene for chloramphenicol acetyltransferase (cat) and terminator sequences were introduced downstream of the cat gene. The terminator sequences stop transcription originating on strong PR promoter that would otherwise proceed through the region of replication origin and interfere with plasmid replication. Thus the copy number of recombinant plasmid molecules is stabilized. The cloning system has been constructed in a new YEp type shuttle vector, pPW264. The 8.1 kb-vector carries two unique cloning sites, BglII and HindIII. The maintenance of the vector and selection in yeast is ensured by URA3 Saccharomyces cerevisiae gene. The vector was employed in cloning of the gene for alpha-amylase from Schwanniomyces occidentalis.

Construction of a new Escherichia coli-Saccharomyces cerevisiae shuttle plasmid cloning vector allowing positive selection for cloned fragments

A new E. coli-S. cerevisiae shuttle plasmid cloning vector (pPW263) with a positive type of selection, was constructed. The selection system, based on the regulatory region of lambda phage controlling the expression of tetracycline resistance, was derived from the cloning vector pUN121 (Nilsson et al. 1983). There are three cloning sites in the cI gene, EcoRI, HindIII and BglII, and, in addition, two unique sites in the neighborhood, BamHI and SalI. The size of the vector is 7.8 kb. The maintenance of the vector and the selection in yeast was ensured by the replication region of the 2 mu plasmid and by the URA3 marker gene, respectively.

Plasmid pIJ699, a multi-copy positive-selection vector for Streptomyces

A plasmid vector, pIJ699, which provides positive selection for cloned DNA, was constructed using the replication functions of the Streptomyces wide-host-range multi-copy plasmid pIJ101. The selection for inserts is based on the principle that plasmids with long uninterrupted perfect palindromes (inverted repeats) are 'not viable' in bacteria. For cloning, pIJ699 is digested with BglII. This produces two fragments, one of which is the linearized vector, with two arms of the palindrome at its ends, and the other is a 'spacer' which is needed to keep the inverted repeat sequences apart. The vector fragment is separated from the 'spacer' fragment and ligated with the DNA to be cloned. Plasmids with a fragment of cloned DNA, but not the circularized vector, give rise to thiostrepton-resistant transformants in Streptomyces lividans. The inverted repeat sequences contain a strong transcription terminator which reduces transcriptional read-through both in and out of the cloned fragment. This improves the stability of many hybrid plasmids and facilitates the study of the regulation of cloned genes.