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DNA complementary to the RNA of purified potato leafroll virus (PLRV) was synthesized and cloned into the lambda insertion vector NM1149. Overlapping PLRV-specific cDNA clones were isolated that represent some 80% of the viral genome. Sequences coding for the capsid protein were identified by subcloning size-selected cDNAs into the lambda expression vector gt11 and screening with PLRV-specific antisera. The gene for the viral capsid protein was shown to reside in the 3' terminal half of the genomic RNA. Sequence comparisons with the recently published genomes of the beet western yellows virus (BWYV) and the barely yellow dwarf virus (BYDV) reveal some 65% protein sequence homology between the capsid proteins of BWYV and PLRV and some 45% homology between BYDV and PLRV. Furthermore, it is evident that the structural organization of the PLRV genome in the CP gene region is similar to that of BWYV and BYDV.

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DT-IG, a symptomless coat-protein-free TMV mutant was used to induce a high degree of cross-protection (about 90 %) against UI strain in samsun tobacco. This mutant spreads slowly from cell to cell, so that it is difficult to have all cells infected in the leaf by mechanical inoculation and to achieve lOa % cross-protection. The absence of coat protein in DT-IG infected samsun leaves was proved: I. by silver staining of polyacrylamide gels after separation of total proteins by electrophoresis and 2. ELISA-test with two different enzyme conjugates. Cross-protection effect was quantified by measuring the amount of coat protein produced by the challenging VI strain.The presence of TMV coat protein is not essential for cross-protection. ZusammenfassungDas Hiillprotein von Tabakmosaikvirus spielt keine bedeutende Rolle fur die Pramunitat DT-IG, eine symptomlose, Hiillprotein-freie TMV Mutante wurde benutzt, urn ein hohes MaG an Prarnunirat (ca. 90 %) gegen den UI Stamm (Wildstamm) in Samsun Tabak zu induzieren. Diese Mutante breitet sich langsam von Zelle zu Zelle aus, so daG es schwierig ist, bei mechanischer Inokulation aile Zellen in einem Blatt zu infizieren und eine lOa %ige Prarnunitat zu erreichen. Das Fehlen von Hiillprotein in DT-IG infizierten Samsun Blattern wurde iiberpriift: 1. durch Silberfarbung von Polyacrylamid Gelen nach Auftrennung von gesamtextrahierbarem Protein und 2. ELISA Test mit zwei verschiedenen Enzymkonjugaten. Der Pramunitatseffekt wurde quantifiziert durch Messung von Hiillprotein, das durch den zweirverimpften UI Stamm produziert wurde. Das Vorhandensein von TMV Hiillprotein ist nicht essentiell [iir das .Auslosen der Pramunitat. U.S.

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