Determinants of site-specific recombination in the lambdoid coliphage HK022. An evolutionary change in specificity.

The temperate bacteriophage HK022, like its relative lambda, inserts its chromosome into a specific site in the bacterial chromosome during lysogenization and excises it after induction. However, we find that the recombinational specificities of the two phages differ: they use different bacterial sites, and neither promotes efficient insertion or excision of the other phage chromosome. In order to determine the basis for this difference in specificity, we sequenced the HK022 elements that are involved in insertion and excision, and compared them to the corresponding lambda elements. The location, orientation, size and overall arrangement of the int and xis genes and the phage attachment sites are nearly identical in the two genomes, as is common for other functionally related elements in lambdoid phages. The Xis proteins of the two phages are functionally interchangeable, and their predicted amino acid sequences differ by but one residue. In contrast, the two Int proteins are not functionally interchangeable, and their sequences, although similar, differ at many positions. These sequence differences are not uniformly distributed: the amino-terminal 55 residues are completely conserved, but the remaining 302 show a pattern of differences interspersed with identities and conservative changes. These findings imply that the specificity difference between HK022 and lambda site-specific recombination is a consequence of the inability of the respective Int proteins to recognize pairs of heterologous attachment sites. The two phage attachment sites are remarkably similar, especially the two "arm" segments, which in lambda contain binding sites for Int, Xis and integration host factor. They are less similar in the segment between the two arms, which in lambda contains the points of recombinational strand exchange and a second class of binding site for Int protein (the "core-type" sites). The two bacterial attachment sites are quite different, although both have a short stretch of perfect homology with their respective phage partners at the points of strand exchange. We propose that the two Int proteins recognize similar or identical sites in the arms of their cognate attachment sites, and that differences in binding or action at the core-type sites is responsible for the divergent specificities. Genetic experiments and sequence comparisons suggest that both proteins recognize different but overlapping families of core-type sites, and that divergence in specificity has been achieved by an alternating succession of small, mutually compatible changes in protein and site.

Use of a stationary bed reactor and serum-free medium for the production of recombinant proteins in insect cells.

Insect cells (Spodoptera frugiperda) have been cultured in a stationary bed reactor, packed with a fibrous polyester carrier. When the bioreactor was perfused with serum-supplemented medium, a cell density of 6 x 10(6) cells ml-1 packed carrier was reached. Scanning electron microscopy investigations have shown that the insect cells grew along the three-dimensionally oriented fibers of the Fibra-cel carrier. After infection of the logarithmically growing cells with a recombinant baculovirus (Autographa californica) containing the gene coding for beta-galactosidase, the medium in the bioreactor was changed to serum-free medium. At day 13 postinfection (p.i.), a beta-galactosidase level of 320 microgram ml-1 and, at day 17 p.i., a virus titer of 2.1 x 10(8) TCID50 units ml-1 (day 17 p.i.) were reached. In another bioreactor, operated in a similar way but with serum-containing medium, a beta-galactosidase concentration of 360 microgram ml-1 and a virus titer of 2.3 x 10(8) TCID50 units ml-1 were obtained. These results indicate the potential use of this production system for the production of recombinant protein and baculovirus in insect cells.

DNA homology relationships between Spodoptera littoralis nuclear polyhedrosis virus and other baculoviruses.

The DNA sequence homology relationships between Spodoptera littoralis nuclear polyhedrosis virus (NPV) and five other lepidopteran NPVs were studied by hybridization of 32P-labeled NPV DNAs to Southern blots of restriction-endonuclease-digested NPV DNA. The S. littoralis NPV (SlMNPV) shows extensive DNA sequence homology through most of the genome with S. frugiperda NPV and S. exigua NPV. Heliothis armigera NPV, H. zea NPV, and Autographa californica NPV (AcMNPV) share fewer regions of homology with SlMNPV. Using the cloned HindIII fragment V of AcMNPV DNA, fragments with sequence homology to the polyhedrin gene of AcMNPV were identified for the viruses studied.

Utility of ethidium bromide in the extraction from whole plants of high molecular weight maize DNA.

A procedure is reported for the isolation of high molecular weight maize DNA from whole plant tissue. Nuclei are isolated in the presence of ethidium bromide from leaf, node, and tassel or endosperm tissues and the DNA is extracted and purified. The resulting DNA has a double-strand molecular weight of about 125 kilobase pairs and a single-strand molecular weight of about 125 kilobases. The DNA is cleavable by a number of common restriction endonucleases.

DNA restriction-pattern differences from geographic isolates of Spodoptera littoralis nuclear polyhedrosis virus.

Two nuclear polyhedrosis viruses were isolated from Spodoptera littoralis larvae collected from several areas in Israel. The two viruses were characterized and compared by restriction endonuclease cleavage of their DNA and Southern blot hybridization. The two viruses have no detectable sequence homology. A variant, homologous to the more common virus isolate, was also found.

Ultrastructural changes in the cell walls of ripening apple and pear fruit.

Ultrastructural changes in the cell walls of "Calville de San Sauveur" apples (Malus sylvestris Mill) and "Spadona" pear (Pyrus communis L.) fruit were followed during ripening. In apple, structural alterations in cell walls became apparent at advanced stages of softening and showed predominantly dissolution of the middle lamella. In pears softening was also associated with the dissolution of the middle lamella, and in addition a gradual disintegration of fibrillar material throughout the cell wall. In fully ripe fruit almost all of the fibrillar arrangement in the cell wall was lost. Application of enzyme solutions containing polygalacturonase and cellulase to tissue discs from firm pear fruit led to ultrastructural changes observed in naturally ripening pears. In apple polygalacturonase alone was sufficient to dissolve the middle lamella region of the cell walls, as was also found to occur in naturally ripening fruit. In both apple and pear the cell wall areas containing plasmodesmata maintained their structural integrity throughout the ripening process. At advanced stages of ripening vesicles appeared in the vicinity of plasmodesmata.