Turnip yellow mosaic virus

Two histidines of the coat protein of turnip yellow mosaic virus at the capsid interior are crucial for viability

RNA-coat protein interactions in turnip yellow mosaic virus (TYMV) have been shown to involve low pK proton-donating groups. Two different types of interaction have been proposed. In the so-called type I interaction, protonated C-residues interact with acidic amino acids at low pH, thereby providing a rationale for the high C-content (38%) of the genomic RNA. The type II interaction involves charged histidines interacting with phosphates of the RNA backbone. Site-directed mutagenesis of the TYMV coat protein and subsequent in vivo analysis were performed to distinguish between these two types of RNA-protein interaction. The results reveal a prominent role for the histidines H68 and H180, since mutation to an alanine residue inhibits symptom development on secondary leaves, indicating that spreading of the virus in the plant is blocked. Viral RNA and coat protein synthesis are not altered, showing that these two histidines may play a role in the process of RNA encapsidation. Overexpression of the TYMV coat protein in Escherichia coli leads to the formation of bona fide capsids, showing that the two histidines are not critical in capsid assembly. Mutagenesis of the acidic amino acids D11, E135, and D143 to alanine apparently did not interfere with virus viability. The functional role of the histidines during the infection cycle is discussed in terms of the structure of the coat protein, both at the level of amino acid sequence conservation among the members of the Tymoviridae family and as the three-dimensional structure of the coat protein.

Alkylation of nucleic acids by DNA-targeted 4-anilinoquinolinium aniline mustards: kinetic studies

The rate of constant for hydrolysis of a series of 4-substituted aniline mustards Ar-X-pC6H4-N(CH2CH2Cl)2, where Ar is 4-anilinoquinolinium and X = O, CH2, CONH and CO, have been measured in water and 0.02 M imidazole buffer at 37 degrees C and in 50% aqueous acetone at 66 degrees C. The equilibrium binding constants of the compounds and their hydrolysis products to nucleic acids of differing base composition have been determined at varying ionic strengths, and the results are consistent with the compounds binding as expected in the DNA minor groove. The alkylating reactivity of the mustards towards these nucleic acids has been measured in water at 37 degrees C and in 0.01 M HEPES buffer over a range of temperatures from 25 degrees C to 60 degrees C. Evaluation of the thermodynamic parameters for these kinetic and equilibrium studies suggests that the interaction with nucleic acids is via an internal SN2 mechanism involving an aziridinium ion.

Lipoprotein-X: a substrate for lecithin: cholesterol acyltransferase

The action of lecithin:cholesterol acyltransferase (LCAT) was studied on an abnormal lipoprotein (LP-X) rich in phosphatidylcholine and cholesterol from the plasma of patients with obstructive liver disease. 60 mg LP-X isolated free of other lipoproteins and subsequently labelled with 3H-cholesterol were incubated with 1 mg highly purified enzyme in the presence of albumin. After 45 h at 37 degrees C, the incubation mixture was subjected to zonal ultracentrifugation. 3H-cholesterol and 3H-cholesteryl esters were quantified in each fraction of the zonal gradient. More than 95% of the lipoproteins in this mixture banded in the density range of LP-X with no change in size distribution, but did contain 593 nmoles of newly formed cholesteryl esters. Agarose electrophoresis revealed an alpha-migrating band in addition to the original beta-band. Also on agar, the typically cathode migrating LP-X was changed to anode moving material. These studies indicate that LP-X can serve as a substrate for LCAT.

Translation of turnip yellow mosaic virus RNA in vitro: a closed and an open coat protein cistron

Sucrose gradient centrifugation of heat-denatured RNA of turnip yellow mosaic virus permitted the isolation of five RNA classes with molecular weights ranging from 2.0 to 0.25 X 10(6). The infectivity was shown to be confined to an RNA molecule of molecular weight 2.0 X 10(6). No significant increase in infectivity was obtained by combination of the latter RNA with the RNA classes of smaller size. Translation in vitro of the RNAs of different size classes in a wheat germ cell-free system revealed that the infectious RNA (molecular weight 2.0 X 10(6) does not promote the synthesis of the coat protein of turnip yellow mosaic virus. Efficient production of this coat protein was found exclusively when the smallest RNA class (molecular weight 250,000) was used as a messenger. It is concluded that RNA molecules of turnip yellow mosaic virus of molecular weight 2.0 X 10(6) contain a closed coat protein cistron and that RNA molecules of molecular weight about 2 to 3 X 10(5) with an open coat protein cistron can be isolated from the virions.