Geminiviruses: Models for plant DNA replication, transcription, and cell cycle regulation

Geminiviruses have small, single-stranded DNA genomes that replicate throug h double-stranded intermediates in the nuclei of infected plant cells. Vira l double-stranded DNA also assembles into minichromosomes and is transcribe d in infected cells. Geminiviruses encode only a few proteins for their rep lication and transcription and rely on host enzymes for these processes. Ho wever, most plant cells, which have exited the cell cycle and undergone dif ferentiation, do not contain the replicative enzymes necessary for viral DN A synthesis. To overcome this barrier, geminiviruses induce the accumulatio n of DNA replication machinery in mature plant cells, most likely by modify ing cell cycle and transcriptional controls. In animals, several DNA viruse s depend on host replication and transcription machinery and can alter thei r hosts to create an environment that facilitates efficient viral replicati on. Analysis of these viruses and their proteins has contributed significan tly to our understanding of DNA replication, transcription, and cell cycle regulation in mammalian cells. Geminiviruses have the same potential for pl ant systems. Plants offer many advantages for these types of studies, inclu ding ease of transformation, well-defined cell populations and developmenta l programs, and greater tolerance of cell cycle perturbation and polyploidy . Our knowledge of the molecular and cellular events that mediate geminivir us infection has increased significantly during recent years. The goal of t his review is to summarize recent research addressing geminivirus DNA repli cation and its integration with transcriptional and cell cycle regulatory p rocesses.