Stunting

The molecular weights obtained by this method are averages. This is particularly evident from the situations where additives are present. In these cases, two different kinds of chains result, with those terminated by the same end group being stunted in growth compared to the normal polycaprolactam. Yet it is the total weight of polymer and the total number of ends that are... 

Chemical treatment of diseases is not common, because of legislative controls and costs caused by the difficulty of appHcation through the leaf canopy. Breeding of resistant varieties is the main weapon for disease control. Some diseases, chiefly ratoon stunting disease, are controlled by hot water treatment of cane (6,8). 

Symptoms of deficiency in animals include poor appetite, stunted growth, and weight loss increased incidence of irritabihty and convulsions (tetany) some growth abnormahties decreased egg production in poultry with reduced hatchabihty and thin eggsheU quahty and birth of weak, dead, or deformed offspring in other animals. 

A nucleic acid can never code for a single protein molecule that is big enough to enclose and protect it. Therefore, the protein shell of viruses is built up from many copies of one or a few polypeptide chains. The simplest viruses have just one type of capsid polypeptide chain, which forms either a rod-shaped or a roughly spherical shell around the nucleic acid. The simplest such viruses whose three-dimensional structures are known are plant and insect viruses the rod-shaped tobacco mosaic virus, the spherical satellite tobacco necrosis virus, tomato bushy stunt virus, southern bean mosaic vims. 

The molecular basis for quasi-equivalent packing was revealed by the very first structure determination to high resolution of a spherical virus, tomato bushy stunt virus. The structure of this T = 3 virus was determined to 2.9 A resolution in 1978 by Stephen Harrison and co-workers at Harvard University. The virus shell contains 180 chemically identical polypeptide chains, each of 386 amino acid residues. Each polypeptide chain folds into distinct modules an internal domain R that is disordered in the structure, a region (a) that connects R with the S domain that forms the viral shell, and, finally, a domain P that projects out from the surface. The S and P domains are joined by a hinge region (Figure 16.8). 

Figure 16.8 Architecture of the tomato bushy stunt virus particle. The polypeptide chain of each subunit folds into three domains (R, S, P) with a 35-residue connecting arm (a) between R and S and a hinge (h) between S and P.

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

The N-terminal part of the tomato bushy stunt virus polypeptide chain (the R-segment in Figure 16.8) is disordered in all the subunits. As in the core of many other single-strand RNA viruses this region of the polypeptide chain... 

Figure 16.9 Contacts between P domains in tomato bushy stunt virus. Sa, Sb, and Sc are the shell domains of subunits A, B, and C, respectively. Pa, Pb, and Pc are the protruding domains of subunits A, B, and C, respectively.

Figure 16.10 The arms of all 60 C subunits in tomato bushy stunt virus form an internal framework, (a) Configuration of interdigitated arms from the three C subunits, viewed down a threefold axis. The p strands are shown as arrows, (b) Cutaway view of the virus particle, emphasizing the framework function of the C-subunit arms. These arms are shown as chains of small balls, one per residue. The region where three arms meet and interdigitate is shown schematically in (a). The main part of each subunit is represented by large balls. Only about one hemisphere of these is drawn, but all the C-subunit arms are included.

The asymmetric unit contains one copy each of the subunits VPl, VP2, VP3, and VP4. VP4 is buried inside the shell and does not reach the surface. The arrangement of VPl, VP2, and VP3 on the surface of the capsid is shown in Figure 16.12a. These three different polypeptide chains build up the virus shell in a way that is analogous to that of the three different conformations A, C, and B of the same polypeptide chain in tomato bushy stunt virus. The viral coat assembles from 12 compact aggregates, or pen tamers, which contain five of each of the coat proteins. The contours of the outward-facing surfaces of the subunits give to each pentamer the shape of a molecular mountain the VPl subunits, which correspond to the A subunits in T = 3 plant viruses, cluster at the peak of the mountain VP2 and VP3 alternate around the foot and VP4 provides the foundation. The amino termini of the five VP3 subunits of the pentamer intertwine around the fivefold axis in the interior of the virion to form a p stmcture that stabilizes the pentamer and in addition interacts with VP4. 

One of the most striking results that has emerged from the high-resolution crystallographic studies of these icosahedral viruses is that their coat proteins have the same basic core structure, that of a jelly roll barrel, which was discussed in Chapter 5. This is true of plant, insect, and mammalian viruses. In the case of the picornaviruses, VPl, VP2, and VP3 all have the same jelly roll structure as the subunits of satellite tobacco necrosis virus, tomato bushy stunt virus, and the other T = 3 plant viruses. Not every spherical virus has subunit structures of the jelly roll type. As we will see, the subunits of the RNA bacteriophage, MS2, and those of alphavirus cores have quite different structures, although they do form regular icosahedral shells. 

Figure 16.13 The known subunit structures of plant. Insect, and animal viruses are of the jelly roll antiparallel p barrel type, described in Chapter 5. This fold, which is schematically illustrated in two different ways, (a) and (b), forms the core of the S domain of the subunit of tomato bushy stunt virus (c). [(b), (c) Adapted from A.J. Olson et al., /. Mol. Biol. 171 61-93, 1983.1...

Tomato bushy stunt virus is a T = 3 plant virus with 180 chemically identical subunits. Each polypeptide chain is divided into several domains. The subunits preserve quasi-equivalent packing in most contact regions by conformational differences of the protein chains, especially a large change in... 

Picornaviruses construct their shells from 60 copies each of three different polypeptide chains. These 180 subunits are arranged within the shell in a manner very similar to the 180 identical subunits of bushy stunt virus. In some picornaviruses there are protrusions around the fivefold axes, which are surrounded by deep "canyons." In rhinoviruses, the canyons form the virus s attachment site for protein receptors on the surface of the host cells, and they are adjacent to cavities that bind antiviral drugs. 

Robinson, I.K., Harrison, S.C. Structure of the expanded state of tomato bushy stunt virus. Nature 297 ... 

Verdacht, m. suspicion, distrust, verdachtig, a. suspicious, suspected, verdkmmen, v.t. tamp, dam check stunt. Verdampfapparat, m. evaporating apparatus, evaporator vaporizer carburetor, verdampfbar, a. vaporizable volatile. Verdampf-barkeit, /. vaporizability, volatility. 

Saline soils are often recognized in the field by the presence of a white surface crust, by damp oily-looking surfaces devoid of vegetation, stunted plant growth, and sometimes by tipbum and firing of leaf margins. Soil analysis rather than visual observations are nevertheless needed to properly assess salinity. 

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