Department of Biology, The University of Texas at San Antonio, San Antonio, TX 78249, U.S.A.
Spinach curly top virus (SCTV), the fifth characterized Curtovirus species belonging to the family Geminiviridae, is an agriculturally significant plant pathogen representing an emerging disease threat in the southern United States. The SCTV genome comprises a single DNA chromosome of approximately 3.0 kb, with the potential to code for seven proteins larger than 10 kDa but which relies extensively on the host for replication and transcription of its genome. In this study, we have identified viral and complementary sense transcripts in SCTV-infected plants, confirming a bidirectional transcription strategy for SCTV. The most abundant RNA maps to the virion sense (1.1-kb transcript) and is comparable in size and location to that observed in Beet curly top virus (BCTV). Two complementary sense transcripts (1.7 and 0.7 kb) were identified in SCTV-infected plants. The large, 1.7-kb transcript is comparable in size and position to that identified in BCTV and several begomoviruses and most likely encodes the C1 protein. Both complementary sense RNAs could potentially direct expression of C2 and C3 from polycistronic mRNAs. A mutation in the C2 gene of SCTV results in expression of a truncated protein of 38 amino acids that is capable of interacting with two cellular kinases, AKIN11 and ADK2, and the resulting mutant virus remains highly infectious. A second mutant virus can only express the first three amino acids of the C2 protein and is unable to interact with the same kinases. However, this mutant virus still remains infectious, although a reduction in infectivity and symptom severity was seen in both Arabidopsis and spinach. A possible relationship between the interaction of C2 with AKIN11 and ADK2 and disease severity is presented.