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Giant viruses, with virions larger than 200 nm and genomes larger than 340 kilobase pairs, modified the now outdated perception of the virosphere. With virions now reported reaching up to 1.5 μm in size and genomes of up to 2.5 Mb encoding components shared with cellular life forms, giant viruses exhibit a complexity similar to microbes, such as bacteria and archaea. Here, we review interactions of giant viruses with their hosts and defense strategies of giant viruses against their hosts and coinfecting microorganisms or virophages. We also searched by comparative genomics for homologies with proteins described or suspected to be involved in defense mechanisms. Our search reveals that natural immunity and apoptosis seem to be crucial components of the host defense against giant virus infection. Conversely, giant viruses possess methods of hijacking host functions to counteract cellular antiviral responses. In addition, giant viruses may encode other unique and complex pathways to manipulate the host machinery and eliminate other competing microorganisms. Notably, giant viruses have evolved defense mechanisms against their virophages and they might trigger defense systems against other viruses through sequence integration. We anticipate that comparative genomics may help identifying genes involved in defense strategies of both giant viruses and their hosts. [ABSTRACT FROM AUTHOR]

Viroids are minute unencapsidated non-coding circular RNAs known to be present and to cause diseases only in plants. Infections were associated with the occurrence of specific single-stranded RNAs similar in size to miRNAs and endogenous small interfering RNAs, and viroid pathogenicity is suspected to occur through RNA interference. We looked for sequence similarities between viroids and the seed region of human microRNAs (hsa-miRNAs). Viroid genomes were retrieved from GenBank and mature hsa-mi-RNAs were retrieved from miRBase. Two hundred 300-nucleotide-long sequences were randomly generated as controls. BLAST searches were performed using viroids as queries and hsa-miRNAs as subjects with relaxed parameters, and matches involving hsa-miRNA seed regions were considered. A total of 81,021 matches were found, and 1,501 that showed 100% identity with whole hsa-miRNA seed regions were selected. The most frequent matches involved Chrysanthemum stunt viroid or Hop stunt viroidspecies with hsa-miR-4286, in 365 and 207 cases, respectively. Three hsa-mi-RNAs (miR-4286, miR-6808-5p, and miR-3622a-3p) were involved in 47% of all matches between viroids and hsa-mi-RNAs. Taken together, these findings warrant further investigation on the potential of viroids and their derived small RNAs to cross kingdoms and interact with nucleic acids in humans. [ABSTRACT FROM AUTHOR]