Analysis of a comprehensive dataset of diversity generating retroelements generated by the program DiGReF.

BACKGROUND: Diversity Generating Retroelements (DGRs) are genetic cassettes that can introduce tremendous diversity into a short, defined region of the genome. They achieve hypermutation through replacement of the variable region with a strongly mutated cDNA copy generated by the element-encoded reverse transcriptase. In contrast to "selfish" retroelements such as group II introns and retrotransposons, DGRs impart an advantage to their host by increasing its adaptive potential. DGRs were discovered in a bacteriophage, but since then additional examples have been identified in some bacterial genomes. RESULTS: Here we present the program DiGReF that allowed us to comprehensively screen available databases for DGRs. We identified 155 DGRs which are found in all major classes of bacteria, though exhibiting sporadic distribution across species. Phylogenetic analysis and sequence comparison showed that DGRs move between genomes by associating with various mobile elements such as phages, transposons and plasmids. The DGR cassettes exhibit high flexibility in the arrangement of their components and easily acquire additional paralogous target genes. Surprisingly, the genomic data alone provide new insights into the molecular mechanism of DGRs. Most notably, our data suggest that the template RNA is transcribed separately from the rest of the element. CONCLUSIONS: DiGReF is a valuable tool to detect DGRs in genome data. Its output allows comprehensive analysis of various aspects of DGR biology, thus deepening our understanding of the role DGRs play in prokaryotic genome plasticity, from the global down to the molecular level.

Dual roles for the Mss116 cofactor during splicing of the ai5γ group II intron.

The autocatalytic group II intron ai5γ from Saccharomyces cerevisiae self-splices under high-salt conditions in vitro, but requires the assistance of the DEAD-box protein Mss116 in vivo and under near-physiological conditions in vitro. Here, we show that Mss116 influences the folding mechanism in several ways. By comparing intron precursor RNAs with long (∼300 nt) and short (∼20 nt) exons, we observe that long exon sequences are a major obstacle for self-splicing in vitro. Kinetic analysis indicates that Mss116 not only mitigates the inhibitory effects of long exons, but also assists folding of the intron core. Moreover, a mutation in conserved Motif III that impairs unwinding activity (SAT → AAA) only affects the construct with long exons, suggesting helicase unwinding during exon unfolding, but not in intron folding. Strong parallels between Mss116 and the related protein Cyt-19 from Neurospora crassa suggest that these proteins form a subclass of DEAD-box proteins that possess a versatile repertoire of diverse activities for resolving the folding problems of large RNAs.

Determinants for DNA target structure selectivity of the human LINE-1 retrotransposon endonuclease.

The human LINE-1 endonuclease (L1-EN) is the targeting endonuclease encoded by the human LINE-1 (L1) retrotransposon. L1-EN guides the genomic integration of new L1 and Alu elements that presently account for approximately 28% of the human genome. L1-EN bears considerable technological interest, because its target selectivity may ultimately be engineered to allow the site-specific integration of DNA into defined genomic locations. Based on the crystal structure, we generated L1-EN mutants to analyze and manipulate DNA target site recognition. Crystal structures and their dynamic and functional analysis show entire loop grafts to be feasible, resulting in altered specificity, while individual point mutations do not change the nicking pattern of L1-EN. Structural parameters of the DNA target seem more important for recognition than the nucleotide sequence, and nicking profiles on DNA oligonucleotides in vitro are less well defined than the respective integration site consensus in vivo. This suggests that additional factors other than the DNA nicking specificity of L1-EN contribute to the targeted integration of non-LTR retrotransposons.

The kinetics of ribozyme cleavage: a tool to analyze RNA folding as a function of catalysis.

As catalytically active RNAs, ribozymes can be characterized by kinetic measurements similar to classical enzyme kinetics. However, in contrast to standard protein enzymes, for which reactions can usually be started by mixing the enzyme with its substrate, ribozymes are typically self-cleaving. The reaction has to be initiated by folding the RNA into its active conformation. Thus, ribozyme kinetics are influenced by both folding and catalytic components and often enable indirect observation of RNA folding. Here, I describe how to obtain quantitative ribozyme cleavage data via denaturing polyacrylamide gel electrophoresis (PAGE) of radioactively labeled in vitro transcripts and discuss general considerations for subsequent kinetic analysis.

The low incidence of diversity-generating retroelements in sequenced genomes.

The insertion of a retrotransposable element is usually associated with adverse or, at best, neutral effects on the host. Diversity-generating retroelements (DGRs) are the first elements that seem to offer a direct selective advantage to their phage or prokaryote host by exact replacement of a short, defined region of a host gene with a hypermutated variant. In a previous study, we presented the software DiGReF for identification of DGRs in genome sequences, and compiled the first comprehensive set of diversity-generating retroelements in public databases. We identified 155 elements in more than 6000 prokaryotic and phage genomes, which was a surprisingly low number. In this commentary, we will discuss the low incidence of these elements and speculate about the biological role of bacterial DGRs.

Protein-facilitated ribozyme folding and catalysis.

In vivo, large RNAs rely on proteins to fold to their native conformation. In the case of the S. cerevisiae group II intron ai5 gamma, the DEAD-box protein Mss116 has been shown to promote the formation of the catalytically active structure. However, it is a matter of debate whether it does this by stabilizing on-pathway intermediates or by disrupting misfolded structures. Here we present the available experimental evidence to distinguish between those mechanisms and discuss the possible interpretations.

Group II introns: structure, folding and splicing mechanism.

Group II introns are large autocatalytic RNAs found in organellar genomes of plants and lower eukaryotes, as well as in some bacterial genomes. Interestingly, these ribozymes share characteristic traits with both spliceosomal introns and non-LTR retrotransposons and may have a common evolutionary ancestor. Furthermore, group II intron features such as structure, folding and catalytic mechanism differ considerably from those of other large ribozymes, making group II introns an attractive model system to gain novel insights into RNA biology and biochemistry. This review explores recent advances in the structural and mechanistic characterization of group II intron architecture and self-splicing.

A DEAD protein that activates intron self-splicing without unwinding RNA.

The group II intron ai5gamma from S. cerevisiae requires high temperature and salt to self-splice in vitro, but it is assisted by the protein Mss116 in vivo. Here we show that Mss116 can stimulate splicing of ai5gamma under near-physiological conditions in vitro, which represents one of the first cases in which a DExH/D protein is shown to act on its natural target. Importantly, we demonstrate that a small subset of DEAD-box proteins can also stimulate ai5gamma splicing in vitro and may represent a distinct subfamily of DEAD-box proteins that functions in RNA tertiary structure assembly. Mutational analysis shows that while ATPase activity is required for stimulation of splicing by Mss116, helicase activity is not. This finding indicates that Mss116 is unlikely to promote intron splicing through the unwinding of kinetic traps. Rather, we propose that Mss116 promotes the ordered assembly of large RNA molecules through stabilization of on-pathway intermediates.

Analysis of 5' junctions of human LINE-1 and Alu retrotransposons suggests an alternative model for 5'-end attachment requiring microhomology-mediated end-joining.

Insertion of the human non-LTR retrotransposon LINE-1 (L1) into chromosomal DNA is thought to be initiated by a mechanism called target-primed reverse transcription (TPRT). This mechanism readily accounts for the attachment of the 3'-end of an L1 copy to the genomic target, but the subsequent integration steps leading to the attachment of the 5'-end to the chromosomal DNA are still cause for speculation. By applying bioinformatics to analyze the 5' junctions of recent L1 insertions in the human genome, we provide evidence that L1 uses at least two distinct mechanisms to link the 5'-end of the nascent L1 copy to its genomic target. While 5'-truncated L1 elements show a statistically significant preference for short patches of overlapping nucleotides between their target site and the point of truncation, full-length insertions display no distinct bias for such microhomologies at their 5'-ends. In a second genome-wide approach, we analyzed Alu elements to examine whether these nonautonomous retrotransposons, which are thought to be mobilized through L1 proteins, show similar characteristics. We found that Alu elements appear to be predominantly integrated via a pathway not involving overlapping nucleotides. The results indicate that a cellular nonhomologous DNA end-joining pathway may resolve intermediates from incomplete L1 retrotransposition events and thus lead to 5' truncations.