THE CONCERTED EVOLUTION OF 5S RIBOSOMAL GENES LINKED TO THE REPEAT UNITS OF OTHER MULTIGENE FAMILIES

We review all instances in which the nuclear 5S rRNA genes of fungi, p rotist, nematode, and arthropod species have been reported to be linke d to the tandemly repeated units of the rDNA, trans-spliced leader, an d histone multigene families. The evolution of these gene arrangements is analyzed by mapping them to independently derived phylogenies. The se analyses show that 5S rRNA genes have repeatedly become linked to d iverse tandemly repeated gene families and that such linkages have als o been subsequently inverted or lost in some species. These variable g ene linkages are probably the result of stochastic gains and losses of variant repeat units, where functional 5S rRNA had transposed, by the mechanisms which are responsible for the concerted evolution of tande mly repeated multigene families. We discuss the possible mechanisms of 5S rRNA gene transposition and suggest that the characteristics of th eir promoter elements, transcription, and termination signals may allo w functional copies of these genes to be fortuitously transposed throu gh an RNA intermediate. We also review the evidence which shows that t he linked 5S rRNA gene copies are transcribed. We conclude that the ob served patterns of 5S rRNA gene linkages to the repeat units of other tandemly repeated multigene families have likely arisen due to fortuit ous recombination events and are unlikely to represent the remnants of an eubacterial-like arrangement of rDNA operons or to have been estab lished due to selective pressures.