IDENTIFYING BICISTRONIC SEGMENTS IN THE GENOME OF THE AGENT CAUSING THE UMAL DE RIO CUAF~Q" (MRCV) DISEASE L CONCI1; F GUZMAN1; M del VAS2; A DISTAFANO2; I LAGUNA1 and H HOPP2 1IFFIVE-INTA, Camino 60 cuadras Km. 51A (5119), Argentina: 2IBM-CICV, INTA-Castelar, CC 77 (1708), Moran-Argentina. Background and Objectives The agent causing the "Mal de Rio Cuarto" (MRCV) disease is a reovirus which severely affects maize crops in Argentina. It produces losses ranging between 10 and 100 million dollars per year, depending on the agricultural period. Infected maize plants show symptoms such as internode shortening, dwarfing, total or partial atrophy of both male and female flowers, and concomitant reductions or even total suppression of grain yield. The unequivocal symptom of virus infection is an overgrowth of tissue on the veins in the back of the leaves, termed "enation". The severity of symptoms is associated with plant genotype, age at which infection occurs and weather conditions. Even though the virus is capable of infecting not only maize but also other crops of economic importance such as sorghum, oat, wheat, barley, millet and rye, the damage to these crops is still to be assessed. The MRCV has also been found in a number of weeds, mostly Gramineae which not only operate as a reservoir for the virus from year to year, but also provide shelter for a planthopper called Delphacodes kuscheli Fennah (Homoptera, Delphacidae) which has currently been identified to be the MRCV vector. Initial research on the MRCV disease led to the assumption that it was caused by a filivirus bearing great similarity to both MRDV [1] observed in the Mediterranean area and RBSDV detected in rice crops in southeast Asia. This virus genus shows a genome divided into 10 RNAds segments within a complex capsid consisting of at least 8 polypeptides. Some biological features as well as serological and molecular hybridization tests suggest that it is either different from or distantly related to the fijiviruses mentioned above. Results and Conclusions
Part of the virus genome has been cloned and nearly 25% sequenced (approximately 7OOOnt) including parts of some of the 10 genomic Segments (S). The sequencing of I OO3bp of virus 87 (50% of the segment) was performed and an homology on the nucleotide level of 58.5% to MRDV 86 and 57.5% to RBSDV 87 was confirmed. This homology rose to 65% when the derived aminoacid sequence was analyzed. The presence of two encoding regions separated by a 52bp intercistronic region could be determined. This size coincides with published information for 56 and 87 of other fijiviruses [2]. Seventy percent (1189nt) of MRCV 89 has been sequenced. This clone comprises an extended encoding region, then a 158bp broad intergenic regin and the beginning of a new region. This fact would also indicate a bicistronic structure for this segment already described in other fijiviruses [2]. Recognition sequences typical of eucaryotic ribosomes dose to the ATG where the second ORF initiates were also identified. An homology on the nucleotide level of 72.5% to MRDV 89 was determined and the aminoacid sequence deduced from it showed an homology of 75 to 88% of the same virus. These results reveal that the MRCV belongs to the F~Mddae family, displaying its typical structure with at least two bicistronic segments [2]. Homology percentages with MRDV and RBSDV ranged from 56.5 to 72.5% confirming that MRCV is an entity either different from or distantly related to the fijiviruses mentioned above, although they could suggest a common origin for all of them. References 1. Mime, R.; Boccardo, G.; Dal B6, E. and Nome, F. 1983. Phytopathology73:1290-1292. 2. Marzachf, C.; Antoniazzi, 8.; d'Aquilio, M and Boccardo, G. 1996. Eu. Journal of Plant Pathol. 102:601-605.