Noris E, Vaira A M, Caciagli P, Masenga V, Gronenborn B, Accotto G P
Istituto di Fitovirologia Applicata, CNR, 10135 Torino, Italy.
J Virol. 1998 Dec;72(12):10050-7. doi: 10.1128/JVI.72.12.10050-10057.1998.
A functional capsid protein (CP) is essential for host plant infection and insect transmission in monopartite geminiviruses. We studied two defective genomic DNAs of tomato yellow leaf curl virus (TYLCV), Sic and SicRcv. Sic, cloned from a field-infected tomato, was not infectious, whereas SicRcv, which spontaneously originated from Sic, was infectious but not whitefly transmissible. A single amino acid change in the CP was found to be responsible for restoring infectivity. When the amino acid sequences of the CPs of Sic and SicRcv were compared with that of a closely related wild-type virus (TYLCV-Sar), differences were found in the following positions: 129 (P in Sic and SicRcv, Q in Sar), 134 (Q in Sic and Sar, H in SicRcv) and 152 (E in Sic and SicRcv, D in Sar). We constructed TYLCV-Sar variants containing the eight possible amino acid combinations in those three positions and tested them for infectivity and transmissibility. QQD, QQE, QHD, and QHE had a wild-type phenotype, whereas PHD and PHE were infectious but nontransmissible. PQD and PQE mutants were not infectious; however, they replicated and accumulated CP, but not virions, in Nicotiana benthamiana leaf discs. The Q129P replacement is a nonconservative change, which may drastically alter the secondary structure of the CP and affect its ability to form the capsid. The additional Q134H change, however, appeared to compensate for the structural modification. Sequence comparisons among whitefly-transmitted geminiviruses in terms of the CP region studied showed that combinations other than QQD are present in several cases, but never with a P129.
功能性衣壳蛋白（CP）对于单分体双生病毒的寄主植物感染和昆虫传播至关重要。我们研究了番茄黄化曲叶病毒（TYLCV）的两个缺陷基因组DNA，即Sic和SicRcv。从田间感染的番茄中克隆的Sic没有感染性，而自发源自Sic的SicRcv具有感染性，但不能通过粉虱传播。发现CP中的单个氨基酸变化是恢复感染性的原因。当将Sic和SicRcv的CP氨基酸序列与密切相关的野生型病毒（TYLCV-Sar）的序列进行比较时，在以下位置发现了差异：129位（Sic和SicRcv中为P，Sar中为Q）、134位（Sic和Sar中为Q，SicRcv中为H）和152位（Sic和SicRcv中为E，Sar中为D）。我们构建了在这三个位置包含八种可能氨基酸组合的TYLCV-Sar变体，并测试了它们的感染性和传播性。QQD、QQE、QHD和QHE具有野生型表型，而PHD和PHE具有感染性但不可传播。PQD和PQE突变体没有感染性；然而，它们在本氏烟草叶片圆片中复制并积累了CP，但没有积累病毒粒子。Q129P替换是一个非保守变化，可能会极大地改变CP的二级结构并影响其形成衣壳的能力。然而，额外的Q134H变化似乎补偿了结构修饰。在所研究的CP区域方面，对粉虱传播的双生病毒进行的序列比较表明，除QQD外的其他组合在几种情况下都存在，但从未出现过P129。
