Repression and derepression of minus-strand synthesis in a plus-strand RNA virus replicon.
Plus-strand viral RNAs contain sequences and structural elements that allow cognate RNA-dependent RNA polymerases (RdRp) to correctly initiate and transcribe asymmetric levels of plus and minus strands during RNA replication. cis-acting sequences involved in minus-strand synthesis, including promoters, enhancers, and, recently, transcriptional repressors (J. Pogany, M. R. Fabian, K. A. White, and P. D. Nagy, EMBO J. 22:5602-5611, 2003), have been identified for many viruses. A second example of a transcriptional repressor has been discovered in satC, a replicon associated with turnip crinkle virus. satC hairpin 5 (H5), located proximal to the core hairpin promoter, contains a large symmetrical internal loop (LSL) with sequence complementary to 3'-terminal bases. Deletion of satC 3'-terminal bases or alteration of the putative interacting bases enhanced transcription in vitro, while compensatory exchanges between the LSL and 3' end restored near-normal transcription. Solution structure analysis indicated that substantial alteration of the satC H5 region occurs when the three 3'-terminal cytidylates are deleted. These results indicate that H5 functions to suppress synthesis of minus strands by sequestering the 3' terminus from the RdRp. Alteration of a second sequence strongly repressed transcription in vitro and accumulation in vivo, suggesting that this sequence may function as a derepressor to free the 3' end from interaction with H5. Hairpins with similar sequence and/or structural features that contain sequence complementary to 3'-terminal bases, as well as sequences that could function as derepressors, are located in similar regions in other carmoviruses, suggesting a general mechanism for controlling minus-strand synthesis in the genus.
Zhang
Guohua
G
Department of Cell Biology and Molecular Genetics, University of Maryland-College Park, College Park, MD 20742, USA.
Zhang
Jiuchun
J
Simon
Anne E
AE
D001483:Base Sequence; D029688:Brassica napus / Q000821:virology*; D019184:Carmovirus / Q000235:genetics* / Q000378:metabolism; D015967:Gene Expression Regulation, Viral; D008969:Molecular Sequence Data; D010942:Plant Viruses / Q000235:genetics / Q000378:metabolism; D012367:RNA, Viral / Q000096:biosynthesis* / Q000235:genetics; D012324:RNA-Dependent RNA Polymerase / Q000235:genetics / Q000378:metabolism; D012093:Replicon; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
D012367:RNA, Viral; D012097:Repressor Proteins; D012324:RNA-Dependent RNA Polymerase