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We have completed the pSC101 sequence. The coding capacities of the newly sequenced regions show the presence of two large open reading frames close to the oriT region. Their size and localization suggest that these polypeptide chains could be involved in the transfer process of pSC101.

The Rep protein of tomato yellow leaf curl Sardinia virus (TYLCSV), a single-stranded DNA virus of plants, is the replication initiator essential for virus replication. TYLCSV Rep has been classified among ATPases associated with various cellular activities (AAA؉ ATPases), in superfamily 3 of small DNA and RNA virus replication initiators whose paradigmatic member is simian virus 40 large T antigen. Members of this family are DNA-or RNA-dependent ATPases with helicase activity necessary for viral replication. Another distinctive feature of AAA؉ ATPases is their quaternary structure, often composed of hexameric rings. TYLCSV Rep has ATPase activity, but the helicase activity, which is instrumental in further characterization of the mechanism of rolling-circle replication used by geminiviruses, has been a longstanding question. We present results showing that TYLCSV Rep lacking the 121 N-terminal amino acids has helicase activity comparable to that of the other helicases: requirements for a 3 overhang and 3-to-5 polarity of unwinding, with some distinct features and with a minimal AAA؉ ATPase domain. We also show that the helicase activity is dependent on the oligomeric state of the protein.Initiation of DNA replication is a highly specific and controlled event which depends on the coordinated assembly of large protein complexes at the origin of replication. This process, which is universal, is activated by recognition and binding of the initiator protein to its cognate replication origin on the chromosome. Other steps follow, such as melting at the origin of replication and further unwinding carried out by replicative helicases recruited at the site of replication (31). In the case of extrachromosomal elements, such as viruses and plasmids, the initiator protein is often multifunctional and performs several of these steps, as well as interacting with host factors necessary for replication (42).The Rep protein is the initiator protein of tomato yellow leaf curl Sardinia virus (TYLCSV), a begomovirus of the Geminiviridae family of plant single-stranded DNA (ssDNA) viruses. The Rep proteins of geminiviruses are closely related and show substantial sequence conservation. Rep, also named C1 and AL1, is a multifunctional protein and the only viral protein absolutely required for virus replication. Four functional domains have been delineated for begomovirus Rep: the N-terminal domain (amino acids [aa] 1 to 120), which is involved in initiation of the rolling-circle replication (RCR) utilized by geminiviruses (4,34,35); the oligomerization domain (aa 121 to 180), leading to interactions with itself (34) and with host factors (17); the ATPase domain (aa 181 to 330), which is characterized by the presence of a P loop (discussed in more detail below); and a carboxyl-terminal domain (aa 331 to 359) of unknown function but shown to be required for viral replication in the case of tomato golden mosaic virus (35) (Fig. 1).The ATPase domain of geminivirus Rep proteins was identified as a common element among proteins encoded by...