RESUMO
The inclusion of Mimiviridae members in the putative monophyletic nucleocytoplasmic large DNA virus (NCLDV) group is based on genomic and phylogenomic patterns. This shows that, along with other viral families, they share a set of genes known as core or "hallmark genes," including the gene for the major capsid protein (MCP). Although previous studies have suggested that the maturation of mimivirus MCP transcripts is dependent on splicing, there is little information about the processing of this transcript in other mimivirus isolates. Here we report the characterization of a new mimivirus isolate, called Kroon virus (KV) mimivirus. Analysis of the structure, synteny, and phylogenetic relationships of the MCP genes in many mimivirus isolates revealed a remarkable variation at position and types of intronic and exonic regions, even for mimiviruses belonging to the same lineage. In addition, sequencing of KV and Acanthamoeba polyphaga mimivirus (APMV) MCP transcripts has shown that inside the family, even related giant viruses may present different ways to process the MCP mRNA. These results contribute to the understanding of the genetic organization and evolution of the MCP gene in mimiviruses.IMPORTANCE Mimivirus isolates have been obtained by prospecting studies since 2003. Based on genomic and phylogenomic studies of conserved genes, these viruses have been clustered together with members of six other viral families. Although the major capsid protein (MCP) gene is an important member of the so-called "hallmark genes," there is little information about the processing and structure of this gene in many mimivirus isolates. In this work, we have analyzed the structure, synteny, and phylogenetic relationships of the MCP genes in many mimivirus isolates; these genes showed remarkable variation at position and types of intronic and exonic regions, even for mimiviruses belonging to the same lineage. These results contribute to the understanding of the genetic organization and evolution of the MCP gene in mimiviruses.
Assuntos
Proteínas do Capsídeo/genética , Evolução Molecular , Regulação Viral da Expressão Gênica , Mimiviridae/genética , Splicing de RNA , Transcrição Gênica , Genoma Viral , Mimiviridae/classificação , Mimiviridae/isolamento & purificação , Mimiviridae/ultraestrutura , Filogenia , RNA Viral , Replicação Viral , Microbiologia da ÁguaRESUMO
For many years, gene expression in the three cellular domains has been studied in an attempt to discover sequences associated with the regulation of the transcription process. Some specific transcriptional features were described in viruses, although few studies have been devoted to understanding the evolutionary aspects related to the spread of promoter motifs through related viral families. The discovery of giant viruses and the proposition of the new viral order Megavirales that comprise a monophyletic group, named nucleo-cytoplasmic large DNA viruses (NCLDV), raised new questions in the field. Some putative promoter sequences have already been described for some NCLDV members, bringing new insights into the evolutionary history of these complex microorganisms. In this review, we summarize the main aspects of the transcription regulation process in the three domains of life, followed by a systematic description of what is currently known about promoter regions in several NCLDVs. We also discuss how the analysis of the promoter sequences could bring new ideas about the giant viruses' evolution. Finally, considering a possible common ancestor for the NCLDV group, we discussed possible promoters' evolutionary scenarios and propose the term "MEGA-box" to designate an ancestor promoter motif ('TATATAAAATTGA') that could be evolved gradually by nucleotides' gain and loss and point mutations.
Assuntos
Vírus Gigantes/genética , Regiões Promotoras Genéticas , Transcrição GênicaRESUMO
Acanthamoeba are natural hosts for giant viruses and their life cycle comprises two stages: a trophozoite and a cryptobiotic cyst. Encystment involves a massive turnover of cellular components under molecular regulation. Giant viruses are able to infect only the trophozoite, while cysts are resistant to infection. Otherwise, upon infection, mimiviruses are able to prevent encystment. This review highlights the important points of Acanthamoeba and giant virus interactions regarding the encystment process. The existence of an acanthamoebal non-permissive cell for Acanthamoeba polyphaga mimivirus, the prototype member of the Mimivirus genus, is analyzed at the molecular and ecological levels, and compared to a similar phenomenon previously described for Emiliana huxleyi and its associated phycodnaviruses: the 'Cheshire Cat' escape strategy.