Structural mechanism of bridge RNA-guided recombination.

Insertion sequence (IS) elements are the simplest autonomous transposable elements found in prokaryotic genomes1. We recently discovered that IS110 family elements encode a recombinase and a non-coding bridge RNA (bRNA) that confers modular specificity for target DNA and donor DNA through two programmable loops2. Here we report the cryo-electron microscopy structures of the IS110 recombinase in complex with its bRNA, target DNA and donor DNA in three different stages of the recombination reaction cycle. The IS110 synaptic complex comprises two recombinase dimers, one of which houses the target-binding loop of the bRNA and binds to target DNA, whereas the other coordinates the bRNA donor-binding loop and donor DNA. We uncovered the formation of a composite RuvC-Tnp active site that spans the two dimers, positioning the catalytic serine residues adjacent to the recombination sites in both target and donor DNA. A comparison of the three structures revealed that (1) the top strands of target and donor DNA are cleaved at the composite active sites to form covalent 5'-phosphoserine intermediates, (2) the cleaved DNA strands are exchanged and religated to create a Holliday junction intermediate, and (3) this intermediate is subsequently resolved by cleavage of the bottom strands. Overall, this study reveals the mechanism by which a bispecific RNA confers target and donor DNA specificity to IS110 recombinases for programmable DNA recombination.

Expression of the ASYMMETRIC LEAVES2 gene in the adaxial domain of Arabidopsis leaves represses cell proliferation in this domain and is critical for the development of properly expanded leaves.

The ASYMMETRIC LEAVES2 (AS2) gene, a member of the AS2/LOB gene family, and the ASYMMETRIC LEAVES1 (AS1) gene of Arabidopsis thaliana participate in the development of a symmetrical, expanded lamina. We report here the patterns of expression of these genes, and the importance of the sites of such expression in leaf development. Transcripts of both genes accumulated in the entire leaf primordia at early stages, but the patterns of accumulation changed as the leaves expanded. AS2 and AS1 transcripts were detected, respectively, in the adaxial domain and in the inner domain between the adaxial and abaxial domains of leaves. The ratios of numbers of adaxial cells to abaxial cells in cotyledons of corresponding mutant lines were greater than the ratios in wild-type cotyledons. The low levels of ectopic expression of AS2 under the control of the AS1 promoter in as2 mutant plants restored an almost normal phenotype in some cases, but also resulted in flatter leaves than those of wild-type plants. Strong expression of the construct in wild-type and as2 plants, but not as1 plants, resulted in the formation of narrow, upwardly curled leaves. Our results indicate that AS2 represses cell proliferation in the adaxial domain in the presence of AS1, and that adaxial expression of AS2 at an appropriate level is critical for the development of a symmetrical, expanded lamina. Real-time RT-PCR analysis revealed that mutation of either AS2 or AS1 resulted in an increase in the levels of transcripts of ETTIN (ETT; also known as AUXIN RESPONSE FACTOR3, ARF3) and KANADI2 (KAN2), which are abaxial determinants, and YABBY5 (YAB5). Thus, AS2 and AS1 might negatively regulate the expression of these genes in the adaxial domain, which might be related to the development of flat and expanded leaves.

The ASYMMETRIC LEAVES2 gene of Arabidopsis thaliana, required for formation of a symmetric flat leaf lamina, encodes a member of a novel family of proteins characterized by cysteine repeats and a leucine zipper.

The ASYMMETRIC LEAVES2 (AS2) gene of Arabidopsis thaliana is involved in the establishment of the leaf venation system, which includes the prominent midvein, as well as in the development of a symmetric lamina. The gene product also represses the expression of class 1 knox homeobox genes in leaves. We have characterized the AS2 gene, which appears to encode a novel protein with cysteine repeats (designated the C-motif) and a leucine-zipper-like sequence in the amino-terminal half of the primary sequence. The Arabidopsis genome contains 42 putative genes that potentially encode proteins with conserved amino acid sequences that include the C-motif and the leucine-zipper-like sequence in the amino-terminal half. Thus, the AS2 protein belongs to a novel family of proteins that we have designated the AS2 family. Members of this family except AS2 also have been designated ASLs (AS2-like proteins). Transcripts of AS2 were detected mainly in adaxial domains of cotyledonary primordia. Green fluorescent protein-fused AS2 was concentrated in plant cell nuclei. Overexpression of AS2 cDNA in transgenic Arabidopsis plants resulted in upwardly curled leaves, which differed markedly from the downwardly curled leaves generated by loss-of-function mutation of AS2. Our results suggest that AS2 functions in the transcription of a certain gene(s) in plant nuclei and thereby controls the formation of a symmetric flat leaf lamina and the establishment of a prominent midvein and other patterns of venation.