Promoters, Transcripts, and Regulatory Proteins of Mungbean Yellow Mosaic Geminivirus

Abstract

Geminiviruses package circular single-stranded DNA and replicate in the nucleus via a double-stranded intermediate. This intermediate also serves as a template for bidirectional transcription by polymerase II. Here, we map promoters and transcripts and characterize regulatory proteins of Mungbean yellow mosaic virusVigna (MYMV), a bipartite geminivirus in the genus Begomovirus. The following new features, which might also apply to other begomoviruses, were revealed in MYMV. The leftward and rightward promoters on DNA-B share the transcription activator AC2-responsive region, which does not overlap the common region that is nearly identical in the two DNA components. The transcription unit for BC1 (movement protein) includes a conserved, leader-based intron. Besides negative-feedback regulation of its own leftward promoter on DNA-A, the replication protein AC1, in cooperation with AC2, synergistically transactivates the rightward promoter, which drives a dicistronic transcription unit for the coat protein AV1. AC2 and the replication enhancer AC3 are expressed from one dicistronic transcript driven by a strong promoter mapped within the upstream AC1 gene. Early and constitutive expression of AC2 is consistent with its essential dual function as an activator of viral transcription and a suppressor of silencing.The family Geminiviridae comprises small circular singlestranded DNA viruses that cause severe diseases in major crop plants worldwide. On the basis of genome organization, host range, and type of insect vector, the family is divided into four genera: Mastrevirus, Curtovirus, Topocuvirus, and Begomovirus (42). Members of the largest genus, Begomovirus (10), infect primarily dicotyledonous plants and are transmitted by the whitefly Bemisia tabaci. Many begomoviruses have a bipartite genome, with a DNA-A component encoding all the protein functions necessary for virus replication in a single cell while the DNA-B component provides movement functions required for systemic spread.Transcription regulation in begomoviruses has been extensively studied in both transgenic plants (13,19,23,40,48,60) and protoplast systems (8,9,13,16,17,24,46,47,49,51,52,53,60). Transcription start and poly(A) sites on both DNA-A and -B have been partially or completely mapped for African cassava mosaic virus (ACMV) (54), Tomato golden mosaic virus (TGMV) (19,36,45,50), Abutilon mosaic virus (AbMV) (14), and the monopartite Tomato leaf curl virus (TLCV) (34). However, major gaps in our understanding of the structural organization of begomovirus promoters and transcription units, and of the regulation of transcription by viral proteins, remain. Bidirectional promoters have been identified in the intergenic regions (IGR) of DNA-A and DNA-B, which share a common region (CR) of about 160 to 200 bp. The CR includes all cisacting elements required for DNA replication and a core promoter driving leftward transcription of the AC1 gene encoding the replication-associated protein (Rep) (reviewed in reference 20). AC1 also functions as a negativ...

Functional Modulation of the Geminivirus AL2 Transcription Factor and Silencing Suppressor by Self-Interaction

“…AL2 protein has several distinct features, including a basic region near the N terminus that contains an NLS, a central region that contains a zinc finger-like motif (C-X 1 -C-X 4 -H-X 2 -C), and an acidic C-terminal region that contains the minimal transcription activation domain (6,15,27,37) (Fig. 1).…”

“…AL2-mediated stimulation of these promoters is complex and involves both activation and derepression mechanisms (29,30,32). The 15-kDa AL2 resembles a typical transcription factor in several respects: it has a nuclear localization signal (NLS), a zinc finger-like domain composed of cysteine and histidine residues, and an acidic activation domain (6,15,27,37). However, dsDNA-binding activity is weak and not sequence specific, and AL2 likely is targeted to responsive promoters by interactions with cellular proteins.…”

Análisis de relaciones estructura-función en el virus del arabesco del Pelargonium.

“…En miembros del género Begomovirus (familia Geminiviridae) se ha propuesto la utilización de los mecanismos de escape al proceso de rastreo ribosomal (que se explica más adelante) y de reiniciación combinados para la síntesis de algunas de sus proteínas (Shivaprasad et al, 2005). Además de en RNAs virales, este mecanismo se ha propuesto también para la traducción de algunos mRNAs celulares que presentan secuencias líder con sORFs que desempeñan un papel en la regulación de la traducción de la(s) ORF(s) localizada(s) aguas abajo (Grant et al, 1995; Morris y Geballe, 2000;Sarrazin et al, 2000; Gaba et al, 2001; Lu et al, 2004;Iacono et al, 2005).…”

“…En miembros del género Begomovirus (familia Geminiviridae) se ha propuesto la utilización de los mecanismos de escape al proceso de rastreo ribosomal (que se explica más adelante) y de reiniciación combinados para la síntesis de algunas de sus proteínas (Shivaprasad et al, 2005 Fütterer y Hohn, 1991;Park et al, 2001).…”

“…Cuando los ribosomas llegan al codón de parada de la sORF, las subunidades ribosomales 40S reinician la síntesis proteica en el siguiente codón de iniciación (Luukkonen et al, 1995). Para que este mecanismo pueda ocurrir, en general es necesario que el tamaño de la sORF no exceda de los 30 codones para evitar en lo posible la disociación de factores esenciales, y que la distancia que separa a la sORF de la ORF principal sea superior a 70 nt para permitir que, después del primer proceso de traducción, la subunidad 40S pueda volver a unirse al complejo eIF2-GTP-Met-tRNA, necesario para el reconocimiento del siguiente codón de inicio y la traducción de la ORF correspondiente (Meijer y Thomas, 2002).En miembros del género Begomovirus (familia Geminiviridae) se ha propuesto la utilización de los mecanismos de escape al proceso de rastreo ribosomal (que se explica más adelante) y de reiniciación combinados para la síntesis de algunas de sus proteínas (Shivaprasad et al, 2005). Además de en RNAs virales, este mecanismo se ha propuesto también para la traducción de algunos mRNAs celulares que presentan secuencias líder con sORFs que desempeñan un papel en la regulación de la traducción de la(s) ORF(s) localizada(s) aguas abajo (Grant et al, 1995; Morris y Geballe, 2000;Sarrazin et al, 2000; Gaba et al, 2001; Lu et al, 2004;Iacono et al, 2005).…”