Hormuzdi S G, Bisaro D M
Biotechnology Center, Ohio State University, Columbus 43210.
Virology. 1995 Feb 1;206(2):1044-54. doi: 10.1006/viro.1995.1027.
The monopartite DNA genome of beet curly top geminivirus (BCTV, strain Logan) contains four leftward (complementary sense) open reading frames (ORFs) designated L1, L2, L3, and L4. We investigated the functions of the L2 and L3 ORFs by mutational analysis. We found that in Nicotiana benthamiana and sugarbeet plants, neither a functional L2 nor a functional L3 gene is required for infectivity. Double mutants were also infectious, and no evidence for a synergistic effect of these genes was evident. However, while sugarbeet plants inoculated with L2 or L3 mutants showed symptoms that were indistinguishable from those elicited by wild type virus, mutant-inoculated N. benthamiana plants displayed a novel phenotype in which recovery of the plant from initially severe disease symptoms was greatly enhanced. Enhanced recovery was associated with a large reduction in viral DNA levels. Our studies did not provide evidence for functional homology between the BCTV L2 gene and its presumed homologue (AL2) in the bipartite geminiviruses. In contrast, mutants with lesions in the L3 ORF accumulated three- to five-fold less DNA than wild type virus in a protoplast replication assay, consistent with the interpretation that the BCTV L3 gene is a homologue of the bipartite geminivirus AL3 gene which is known to function as a replication enhancer. Functional homology was directly confirmed in experiments which demonstrated that the BCTV L3 gene can complement a tomato golden mosaic virus AL3 mutant, and vice versa.
甜菜曲顶双生病毒（BCTV，洛根株系）的单分体DNA基因组包含四个向左（互补链）开放阅读框（ORF），分别命名为L1、L2、L3和L4。我们通过突变分析研究了L2和L3开放阅读框的功能。我们发现，在本氏烟草和甜菜植株中，感染性并不需要功能性的L2或L3基因。双突变体也具有感染性，并且没有明显证据表明这些基因存在协同效应。然而，虽然接种L2或L3突变体的甜菜植株表现出与野生型病毒引发的症状无法区分的症状，但接种突变体的本氏烟草植株表现出一种新的表型，即植株从最初严重的疾病症状中恢复的能力大大增强。恢复增强与病毒DNA水平的大幅降低有关。我们的研究没有提供证据表明BCTV L2基因与其在双分体双生病毒中假定的同源物（AL2）之间存在功能同源性。相比之下，在原生质体复制试验中，L3开放阅读框有损伤的突变体积累的DNA比野生型病毒少三到五倍，这与BCTV L3基因是双分体双生病毒AL3基因的同源物这一解释一致，已知AL3基因起复制增强子的作用。在实验中直接证实了功能同源性，这些实验表明BCTV L3基因可以互补番茄金黄花叶病毒AL3突变体，反之亦然。
