Baranwal Virendra K, Sharma Susheel K, Khurana Deepti, Verma Raj
Advanced Centre for Plant Virology, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi, 110012, India,
Virus Genes. 2014 Feb;48(1):120-7. doi: 10.1007/s11262-013-0984-1. Epub 2013 Oct 8.
Electron microscopy and sequencing of reverse transcriptase and ribonuclease H (RT/RNase H) region of Badnavirus genome from two banana cultivars: Poovan (triploid: AAB) and Safed velchi (diploid: AB), exhibiting leaf streak symptoms, confirmed the association of Banana streak OL virus (BSOLV). As per ICTV species demarcation threshold of 80 % identity in RT/RNase H region, both the isolates were identified as BSOLV. Rolling circle and end-to-end amplification showed the association of two short episomal BSOLV variants: BSOLV-IN1 and BSOLV-IN2 from Poovan and Safed velchi banana, respectively. The genome sizes of both isolates were 6,950 nucleotides long, but shorter than the typical BSOLV genome of 7,389 bp. Open reading frames (ORFs) 1 and 2 of shorter BSOLV isolates shared almost complete nucleotide identity (>99 %) to that of BSOLV. However, the ORF 3 (5,130 bp) and intergenic region (IGR), 886 bp, showed deletions compared with ORF 3 (5,499 bp) and IGR (956 bp) of BSOLV. In phylogenetic analysis for ORF 3 polyprotein, both the isolates clustered with BSOLV, Banana streak CA virus (BSCAV), and Sugarcane bacilliform GA virus (SCBGAV). Identical ORF 1, ORF 2, and the presence of all the conserved domains in short ORF 3 and promoter elements in IGR indicated that these isolates represent replicationally competent shorter variants of BSOLV. These two shorter-than-BSOLV genome sequences and two other identical banana streak virus sequences in GenBank (BSV-TRY; DQ859899 and BSV-GD; DQ451009) might have evolved due to error-prone reverse transcription and splicing or excision from the integrated sequences by homologous recombination in natural banana hybrids under field conditions.
对两个表现出叶条纹症状的香蕉品种（Poovan，三倍体：AAB；Safed velchi，二倍体：AB）的香蕉坏疽病毒基因组的逆转录酶和核糖核酸酶H（RT/RNase H）区域进行电子显微镜观察和测序，证实了香蕉条纹OL病毒（BSOLV）的存在。根据国际病毒分类委员会（ICTV）在RT/RNase H区域80%同一性的物种划分阈值，这两个分离株均被鉴定为BSOLV。滚环扩增和端对端扩增显示分别与Poovan和Safed velchi香蕉中两个短的游离型BSOLV变体相关联：BSOLV-IN1和BSOLV-IN2。两个分离株的基因组大小均为6950个核苷酸长，但短于典型的7389 bp的BSOLV基因组。较短的BSOLV分离株的开放阅读框（ORF）1和2与BSOLV的开放阅读框1和2几乎具有完全的核苷酸同一性（>99%）。然而，与BSOLV的ORF 3（5499 bp）和基因间隔区（IGR，956 bp）相比，ORF 3（5130 bp）和886 bp的基因间隔区存在缺失。在对ORF 3多蛋白的系统发育分析中，两个分离株均与BSOLV、香蕉条纹CA病毒（BSCAV）和甘蔗杆状GA病毒（SCBGAV）聚类。相同的ORF 1、ORF 2以及短ORF 3中所有保守结构域的存在和IGR中启动子元件的存在表明这些分离株代表具有复制能力且较短的BSOLV变体。这两个比BSOLV基因组短的序列以及GenBank中另外两个相同的香蕉条纹病毒序列（BSV-TRY；DQ859899和BSV-GD；DQ451009）可能是由于在田间条件下天然香蕉杂种中易出错的逆转录以及通过同源重组从整合序列中剪接或切除而进化而来。
