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Very small mobile repeated elements in cyanobacterial genomes.

Jeff Elhai1, Michiko Kato, Sarah Cousins

  • 1Center for the Study of Biological Complexity and the Department of Biology, Virginia Commonwealth University, Richmond, Virginia 23284, USA. elhaij@vcu.edu

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Researchers discovered small dispersed repeats (SDRs) in cyanobacteria, which are likely the smallest mobile genetic elements. These mobile DNA sequences, under 100 nucleotides, offer new insights into genome evolution and plasticity.

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Area of Science:

  • Genomics
  • Molecular Evolution
  • Bioinformatics

Background:

  • Mobile DNA elements like transposons and retrotransposons significantly influence genome evolution.
  • Smaller parasitic DNA sequences derived from larger elements are known, but their origins and mobility are often unclear.
  • Previously, highly repeated sequences under 100 nucleotides were considered nonmobile or of unknown origin.

Purpose of the Study:

  • To investigate the presence and nature of small dispersed repeat (SDR) sequences in the genome of Nostoc punctiforme and related cyanobacteria.
  • To determine if these small repeat sequences exhibit characteristics of mobile genetic elements.
  • To understand the evolutionary significance and genomic context of identified SDR families.

Main Methods:

  • Genome-wide survey of Nostoc punctiforme and related cyanobacteria for small dispersed repeat (SDR) sequences.
  • Bioinformatic analysis to identify SDR families within a specific size range (21-27 nucleotides).
  • Phylogenetic analysis and secondary structure prediction to infer evolutionary relationships and functional characteristics.
  • Comparative genomics to assess the distribution and genomic context of SDR elements across different cyanobacterial species.

Main Results:

  • Identification of eight distinct families of SDR sequences, ranging from 21 to 27 nucleotides in length.
  • Discovery that three SDR families (SDR4, SDR5, SDR6) share a common predicted secondary structure, supported by compensatory mutation patterns.
  • Observation that SDR elements are frequently found within tandem repeats, minitransposons, and specifically within the HIP1 octamer (SDR5).
  • Finding that some SDR elements (SDR1, SDR4) exhibit self-insertion, creating nested structures.
  • Demonstration that SDR elements are largely confined to a specific subgroup of cyanobacteria.
  • Evidence suggesting recent mobility of some SDR elements, likely via RNA intermediates.

Conclusions:

  • SDR sequences represent a novel class of mobile genetic elements, potentially the smallest known.
  • These elements contribute to genome plasticity and evolution within a specific lineage of cyanobacteria.
  • The structural and contextual features of SDRs provide insights into their propagation mechanisms and evolutionary history.