An RGG motif in the Rep protein of Tomato yellow leaf curl Sardinia virus is important for transcriptional repression, nuclear localisation and resistance.

Geminiviruses, seriously threatening several crops worldwide, have a circular single-stranded DNA genome replicating in the nucleus with the aid of the replication associated protein (Rep). C?terminally truncated Reps of 210 or 130 amino acids (Rep-210 and -130) of Tomato yellow leaf curl Sardinia virus (TYLCSV) confer resistance to virus infection by inhibiting virus transcription and replication.These functions are not inhibited if a further terminal 10 aa are deleted (Rep-120; Lucioli et al., 2003). By analysing more than 560 Begomovirus Reps, a conserved RGG motif (aa 124-126) was detected in this 10-aa stretch. RGG motifs are involved in RNA binding, transcriptional repression, protein-protein interaction and nuclear/nucleolar localization. Rep-210 and -130 RGG to SAA mutants were produced. They showed reduced transcriptional inhibitory activity in transfected protoplasts compared to their non-mutated counterparts. Instead, only Rep-130 SAA, but not Rep-210 SAA, had a reduced ability to inhibit virus replication. When overexpressed in transgenic plants, Rep-210 and -130 were immunolocalised in the nucleus and nucleolus, while Rep-120, -130 SAA and -210 SAA barely accumulated above background. Furthermore, transgenic plants expressing Rep-210 SAA and -130 SAA showed reduced resistance to TYLCSV. Thus, the RGG mutation affects different activities of TYLCSV Rep.