Vibrio cholerae the comma-shaped bacterium that is the cause of cholera (q.v.). The V. cholerae genome consists of two circular chromosomes. The larger contains 2.96 mbp, the smaller 1.07 mbp. To- gether they have 3,885 ORFs. The smaller chromo- some contains many genes of unknown function, and it may represent a captured plasmid (q.v.). The large chromosome has the majority of genes for essential cell functions and pathogenicity.
The chol- era toxin is encoded in the genome of a virus that has been integrated into the large chromosome.
See Appendix A, Bacteria, Proteobacteria; Appendix C, 1883, Koch; 2000, Heidelberg et al.; Appendix E. vicariance distribution a discontinuous biogeo- graphical distribution of organisms that previously inhabited a continuous range. The current gaps in the distribution were caused by some extrinsic factor (geologic or climatic). Vicia faba the broad bean; also called the fava bean, the horse bean, the Windsor bean. A plant of- ten used in cytogenetics because its cells contain a small number (N = 6) of large chromosomes.
The semiconservative nature of DNA replication was first demonstrated by analyzing autoradiographs of 3H thy- midine-labeled chromosomes from cells of Vicia root tips. See Appendix C, 1957, Taylor et al.; favism. villus a finger-like projection extending from an epithelium. Such a villus is composed of many cells. Contrast with microvillus. vimentin a 55,000-dalton, cytoskeletal protein commonly found in fibroblasts.
In glial cells it is co- polymerized with an acidic protein of 50,000 dal- tons, while in muscle cells it is combined with des- min (q.v.). vinblastine a vinca alkaloid (q.v.). Vinca alkaloid any one of a number of anticancer drugs isolated from Vinca rosea (q.v.).
The drugs block the division of cancer cells by acting as spindle poisons (q.v.). Like colchicine (q.v.) the Vinca alka- loids bind to tubulin (q.v.) and interfere with the assembly of the microtubules of mitotic spindles. Vinca rosea the Madagascar periwinkle, source of the vinca alkaloids. vincristine a vinca alkaloid (q.v.). vinculin a fibrous protein responsible for anchorage of actin filaments to the inner side of the cell mem- brane. Vinculin is located in patches called adhesion plaques on the cell membrane that are thought to be responsible for intercellular adhesion.
Cells infected with Rous sarcoma virus (q.v.) produce a kinase that phosphorylates the tyrosine residues of vinculin. It is hypothesized that the phosphorylation of vinculin both destabilizes actin linkages (allowing transformed cells to become rounded) and weakens intercellular adhesion (allowing metastasis). viral-specific enzyme any enzyme produced in the host cell after viral infection and encoded by a viral gene. viral transformation See transformation. virion a completed virus particle consisting of a nucleic acid core and a protein coat. viroid a disease-causing agent of plants consisting of a circular, single-stranded RNA molecule typi- cally 270-380 nucleotides long and therefore thou- sands of times smaller than the most diminutive vi- rus.
Since its RNA does not encode proteins, the viroid must rely on host enzymes for its replication.
This occurs in the nucleus of the host cell where the viroid RNA is localized within the nucleolus. Repli- cation occurs by a rolling circle mechanism that gen- erates an oligomeric concatomer that is several times the length of the original viroid. Hammerhead ribo- zymes (q.v.) cleave the concatomer into genomic units, and these are subsequently circularized. Vi- roids are not encapsulated in a protein coat, and their genomes are not integrated into the host ge- nome.
The pathogenic effects of viroids on their host results from the fact that the RNA of the viroid con- tains segments that are complementary to the 7S RNAs of the signal recognition particles (q.v.) of their hosts.
Thus, the viroid behaves like an antisense RNA (q.v.) and blocks the formation and functioning of sig- nal recognition particles.
Typical viroids are the apical stunt and planta macho viroids of tomatoes and the cadang cadang viroid of coconut palms. See Appendix C, 1967, Diener and Raymer; rolling circle, virusoid. virulence the relative ability of an organism to produce disease. virulence plasmids genetically related plasmids found in various Salmonella species and in certain E. coli strains that give the bacteria the ability to invade intestinal cells. virulent phage a phage that causes lysis of the host bacterium. Contrast with temperate phage. virus an ultramicroscopic, obligate, intracellular parasite incapable of autonomous replication. Vi- ruses can reproduce only by entering a host cell and using its translational system.
Viruses are generally classified according to the type of nucleic acid they contain and the morphology of the nucleocapsid