Relaxed circular, covalently closed simian virus 40 DNA molecules were associated with the four histones that are present in virions. In electron micrographs the resulting complexes appear twisted, with globular structures (nucleosomes) along the DNA. Incubation with an untwisting extract converts the twisted complexes to relaxed structures. Extraction of the DNA from the relaxed complexes yields supercoiled molecules. The number of superhelical turns in these molecules corresponds to the number of nucleosomes per DNA molecule in the complexes.In eukaryotic nuclei, the fundamental structure of chromatin fibers appears to be a flexible chain composed of globular particles connected by DNA filaments (1, 2). In these particles, termed nucleosomes (2), about 200 base pairs of DNA are associated with the four histonies F2a,, F2a2, F2b, and F3 (2-7). Such a repeating unit structure can be formed in vitro by association of the four histonies and linear l)NAs (2).In the nucleosomes the DNA is under constraint, since it is compacted about 5-fold compared to its length in the extended double helical form (2). The nature of this constraint can be studied by the association of histones to covalently closed circular DNA molecules, since supercoiling or unwinding of the DNA within the nucleosome (luring its formation would alter the supercoiling of the rest of the molecule (8). In addition, from the known thermodynamic prolerties of superhelical DNAs (8-10), the influence of the degree of superhelicity on the formation of nucleosome structures can provide information on whether the formation of a nucleosome is equivalent to an unwinding or winding of the double helix. Simian virus 40 (SV40) D)NA is particularly attractive for such a study for two reasons. First, two circular covalently closed allomorphic forms of this DNA are available, the superhelical DNA I and the relaxed circular DNA Ir which results from the incubation of DNA I with an untwisting extract (FE) (l1). This extract is thought to introduce a single-strand nick into superhelical DNA and to reseal the nick after the torsional tension in the double helix has been relieved. Second, SV40 DNA and the four histones are associated in vivo anld the complexes can be isolated from virions (12, 13) and from infected cells. In the latter case, the complex appears as a compactecl structure with about 20 nueleosomes (14).We

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In the liver of oviparous vertebrates vitellogenin gene expression is controlled by estrogen. The nucleotide sequence of the 5' flanking region of the Xenopus laevis vitellogenin genes A1, A2, B1 and B2 has been determined. These sequences have been compared to each other and to the equivalent region of the chicken vitellogenin II and apo-VLDLII genes which are also expressed in the liver in response to estrogen. The homology between the 5' flanking region of the Xenopus genes B1 and B2 is higher than between the corresponding regions of the other closely related genes A1 and A2. Four short blocks of sequence homology which are present at equivalent positions in the vitellogenin genes of both Xenopus laevis and chicken are characterized. A short sequence with two-fold rotational symmetry (GGTCANNNTGACC) was found at similar positions upstream of the five vitellogenin genes and is also present in two copies close to the 5' end of the chicken apo-VLDLII gene. The possible functional significance of this sequence, common to liver estrogen-responsive genes, is discussed.

We investigated the amplification and purification of phage preparations with respect to titer, contamination level, stability, and technical affordability. Using various production systems (wave bags, stirred-tank reactors, and Erlenmeyer flasks), we obtained peak titers of 10 9 to 10 10 PFU/ml for T4-like coliphages. Phage lysates could be sterilized through 0.22-m membrane filters without titer loss. Phages concentrated by differential centrifugation were not contaminated with cellular debris or bacterial proteins, as assessed by electron microscopy and mass spectrometry, respectively. Titer losses occurred by high-speed pelleting of phages but could be decreased by sedimentation through a sucrose cushion. Alternative phage concentration methods are prolonged medium-speed centrifugation, strong anion-exchange chromatography, and ultrafiltration, but the latter still allowed elevated lipopolysaccharide contamination. T4-like phages could not be pasteurized but maintained their infectivity titer in the cold chain. In the presence of 10 mM magnesium ions, phages showed no loss of titer over 1 month at 30°C.

Electron microscopic evidence indicates that Simian virus 40 (SV40) minichromosomes extracted from infected cells consist of 20 2 nucleosomes, each containing 190-200 base pairs of DNA. About 50% of the nucleosomes are not close together, but connected by segments of DNA of irregular lengths which correspond to about 1 5 x of the viral genome, irrespective of the ionic strength. Micrococcal nuclease digestion studies show that there is about 200 base pairs of DNA in the biochemical unit of SV40 chromatin. Therefore, the visible internucleosomal DNA of the SV40 minichromosome does not arise from an unfolding of a fraction of the 190-200 base pairs of DNA initially wound in the nucleosome. These results support the chromatin model which proposes that the same DNA length is contained in the nucleosome and the biochemical unit. Results from extensive micrococcal nuclease digestion suggest that an SV40 nucleosome consists of a 'core' containing a DNA segment'of about 135 base pairs associated to a DNA fragment more susceptible to nuclease attack. The addition of histone H1 results in a striking condensation of the SV40 minichromosome, which supports the assumption that histone HI is involved in the folding of chromatin fibers.

Investigation of the chromosomal region downstream of the lacZ gene from Lactobacillus delbrueckii subsp. bulgaricus revealed the presence of a gene (prtB) encoding a proteinase of 1,946 residues with a predicted molecular mass of 212 kDa. The deduced amino acid sequence showed that PrtB proteinase displays significant homology with the N termini and catalytic domains of lactococcal PrtP cell surface proteinases and is probably synthesized as a preproprotein. However, the presence of a cysteine near the histidine of the PrtB active site suggests that PrtB belongs to the subfamily of cysteine subtilisins. The C-terminal region strongly differs from those of PrtP proteinases by having a high lysine content, an imperfect duplication of 41 residues, and a degenerated sequence compared with the consensus sequence for proteins anchoring in the cell walls of gram-positive bacteria. Finally, the product of the truncated prtM-like gene located immediately upstream of the prtB gene seems too short to be involved in the maturation of PrtB.Lactobacillus delbrueckii subsp. bulgaricus is one of the lactic acid bacteria used in industrial milk fermentation. The proteolytic system is essential to ensure rapid growth in milk and supply auxotrophic lactic acid bacteria with amino acids from caseins, the major milk proteins. This system is complex and has been extensively studied in lactococci (21, 35). The first step in milk casein degradation is performed by a cell surface proteinase, PrtP. Two types of PrtP enzymes have been distinguished on the basis of their substrate specificities. PI-type proteinases preferentially cleave ␤-casein, whereas PIII-type enzymes degrade ␣, ␤, and caseins (10). Lactococcal PrtPs are serine proteases and show extensive homology with subtilisins secreted by bacilli. Lactococcal proteinases are synthesized as inactive preproproteins. An N-terminal signal peptide of 33 residues is removed during translocation through the cytoplasmic membrane, and the C terminus remains anchored in the cell envelope. Then, a maturation process mediated by a membrane-located lipoprotein, PrtM, leads to the removal of the pro region (154 residues). The 33-kDa PrtM protein is encoded by a gene located immediately upstream and in the opposite direction of prtP (13, 49). Incubation of lactococci cells in a calcium-free buffer promotes self-digestion of the carboxy terminus of PrtP and its release into the extracellular medium.Lactobacilli have been investigated to a lesser extent but display a high cell surface proteinase activity with substrate specificity differing from that of lactococci (4). Cell surface proteinases have been purified from Lactobacillus paracasei subsp. paracasei NCDO151, L. helveticus CNRZ303, and L89, and L. delbrueckii subsp. bulgaricus CNRZ397 (24, 27, 33, 52). The sequence of the gene encoding proteinase from L. paracasei subsp. paracasei was determined; the deduced amino acid sequence shows 1,902 residues and a high degree of identity (96%) with lactococcal PrtP (15). The presence of a prtM gene ...

A quick headspace GC method for quantification of volatiles was developed, involving only minor sample preparation. Yogurt flavor compounds could be quantified in the micrograms per kilogram to milligrams per kilogram range without any difficulty, despite the complex matrix. Volatiles of traditional acidic and mild, less acidic yogurts were compared, and important differences were found for acetaldehyde, 2,3-butanedione, and 2,3-pentanedione. Concentrations of 2,3-butanedione and 2,3-pentanedione increased 2-3-fold in mild, less acidic yogurts compared to traditional acidic ones. This is due to accumulation of the precursors of the diketones, 2-acetolactate and 2-acetohydroxybutyrate, during fermentation in mild, less acidic yogurt. These precursors are subsequently converted to the corresponding diketones during storage. On the contrary, acetaldehyde formation was reduced in the mild yogurt, due to growth differences between the lactic acid bacteria used for fermentation of the milk. The quantitative results presented in this study validate previous GC sniffing conclusions (Ott et al. J. Agric. Food Chem. 1997, 45, 850-858), showing that yogurt aroma is the superposition of impact flavor compounds generated by fermentation on milk compounds.

In Xenopus laevis four estrogen-responsive genes are expressed simultaneously to produce vitellogenin, the precursor of the yolk proteins. One of these four genes, the gene A2, was sequenced completely, as well as cDNAs representing 75% of the coding region of the gene. From this data the exon-intron structure of the gene was established, revealing 35 exons that give a transcript of 5,619 bp without the poly A-tail. This A2 transcript encodes a vitellogenin of 1,807 amino acids, whose structure is discussed with respect to its function. At the nucleic acid as well as at the protein level no extensive homologies with any sequences other than vitellogenin were observed. Comparison of the amino acid sequence of the vitellogenin A2 molecule with biochemical data obtained from the different yolk proteins allowed us to localize the cleavage products on the vitellogenin precursor as follows: NH2 - lipovitellin I - phosvitin (or phosvette II - phosvette I) - lipovitellin II - COOH.