Clusters of 5S rRNAs in the intergenic region of ubiquitin genes in Tetrahymena pyriformis

Comparative analyses among the Trichomonas vaginalis, Trichomonas tenax, and Tritrichomonas foetus 5S ribosomal RNA genes

The 5S ribosomal RNA (5S rRNA) is an essential component of ribosomes. Throughout evolution, variation is found among 5S rRNA genes regarding their chromosomal localization, copy number, and intergenic regions. In this report, we describe and compare the gene sequences, motifs, genomic copy number, and chromosomal localization of the Trichomonas vaginalis, Trichomonas tenax, and Tritrichomonas foetus 5S rRNA genes. T. vaginalis and T. foetus have a single type of 5S rRNA-coding region, whereas two types were found in T. tenax. The sequence identities among the three organisms are between 94 and 97%. The intergenic regions are more divergent in sequence and size with characteristic species-specific motifs. The T. foetus 5S rRNA gene has larger and more complex intergenic regions, which contain either an ubiquitin gene or repeated sequences. The 5S rRNA genes were located in Trichomonads chromosomes by fluorescent in situ hybridization.

The evolution of 5S ribosomal RNA genes linked to the rDNA units of fungal species

5S ribosomal RNA (rRNA) genes are known to move in and out of various tandemly repeated eukaryotic gene families during evolution. Here, we investigated the organization of 5S rRNA genes linked to the ribosomal DNA (rDNA) units in 147 fungal species using available sequence and genome databanks. Whereas some fungal species have no 5S rRNA genes in their rDNA units, others have one or two 5S rRNA copies linked on the same or the opposite strand. There were at least 13 independent changes during the evolution of fungal species. These include two 5S rRNA genes loss, five 5S rRNA genes inversions and six 5S rRNA genes insertions (including duplications). The lower frequency of 5S rRNA genes loss might be due to the fact that these events are more likely to affect fitness. The maximum time required for 5S rRNA gene organization to change between related species was estimated to be 7.5 millions years based on the sequences of the elongation factor alpha genes of Candida glabrata and Saccharomyces mikatae. This time is much longer than the homogenization time predicted from theoretical and experimental studies and likely reflects the lack of closely related species or strains in our data set.

A tandem array of 5S ribosomal RNA genes in Pythium irregulare

The 5S ribosomal RNA genes of the oomycete Pythium irregulare exist in tandem arrays unlinked to the rDNA repeat unit. A clone with a 9.2-kb insert containing an array of 5S genes was identified in a lambda genomic library and was characterized by restriction mapping and partial sequencing. The array consisted of 9 apparently identical 5S genes and their spacers in tandem, followed by a diverged 5S-like sequence that is likely to be a pseudogene. This gene arrangement, although almost universal in plants and animals, is rare in fungi and protists.