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Cucurbitacins are highly oxygenated triterpenoids characteristic of plants in the family Cucurbitaceae and responsible for the bitter taste of these plants. Fruits of bitter melon (Momordica charantia) contain various cucurbitacins possessing an unusual ether bridge between C5 and C19, not observed in other Cucurbitaceae members. Using a combination of next-generation sequencing and RNA-Seq analysis and gene-to-gene co-expression analysis with the ConfeitoGUIplus software, we identified three P450 genes, CYP81AQ19, CYP88L7, and CYP88L8, expected to be involved in cucurbitacin biosynthesis. CYP81AQ19 co-expression with cucurbitadienol synthase in yeast resulted in the production of cucurbita-5,24-diene-3β,23α-diol. A mild acid treatment of this compound resulted in an isomerization of the C23-OH group to C25-OH with the concomitant migration of a double bond, suggesting that a nonenzymatic transformation may account for the observed C25-OH in the majority of cucurbitacins found in plants. The functional expression of CYP88L7 resulted in the production of hydroxylated C19 as well as C5-C19 ether-bridged products. A plausible mechanism for the formation of the C5-C19 ether bridge involves C7 and C19 hydroxylations, indicating a multifunctional nature of this P450. On the other hand, functional CYP88L8 expression gave a single product, a triterpene diol, indicating a monofunctional P450 catalyzing the C7 hydroxylation. Our findings of the roles of several plant P450s in cucurbitacin biosynthesis reveal that an allylic hydroxylation is a key enzymatic transformation that triggers subsequent processes to produce structurally diverse products.

Somatic plant cells can regenerate shoots and/or roots or adventitious embryonic calluses, which may induce organ formation under certain conditions. Such regenerations occur via dedifferentiation of somatic cells, induction of organs, and their subsequent outgrowth. Despite recent advances in understanding of plant regeneration, many details of shoot induction remain unclear. Here, we artificially induced shoot stem-like green organs (SSOs) in Arabidopsis thaliana roots via simultaneous induction of two transcription factors (TFs), ARABIDOPSIS THALIANA HOMEOBOX PROTEIN 25 (ATHB25, At5g65410) and the B3 family transcription factor REPRODUCTIVE MERISTEM 7 (REM7, At3g18960), which are expressed in shoot apical meristems under normal growth conditions. The SSOs exhibited negative gravitropism and differentiated vascular bundle phenotypes, strongly suggesting the ectopic appearance of shoot stem characteristics in roots. In the SSOs, shoot-specific and root-specific genes were up- and down-regulated, respectively. These TFs can thus induce the expression of genes controlling shoot stem characteristics and disrupt the maintenance of root meristems by ectopic expression in roots. SSO induction appears to occur without development of an embryonic callus in the main root tip or the primordia of lateral roots, suggesting that these TFs alter the fate of root initiation to induce shoot stem characteristics. Our findings thus provide new insights in the development and regeneration of plant shoot stems.

The sizes of the centromeric regions of Arabidopsis thaliana chromosomes 1, 2, and 3 were determined by construction of their physical maps on the basis of restriction analysis. As the reported centromeric regions contain large gaps in the middle due to highly repetitive sequences, appropriate probes for Southern hybridization were prepared from the sequences reported for the flanking regions and from the sequences of BAC and YAC clones newly isolated in this work, and restriction analysis was performed using DNA of a hypomethylated strain (ddm1). The sizes of the genetically defined centromeric regions were deduced to be 9 megabases (Mb), 4.2 Mb and 4.1 Mb, respectively (chromosome 1, from markers T22C23-t7 to T3P8-sp6; chromosome 2, from F5J15-sp6 to T15D9; chromosome 3, from T9G9-sp6 to T15M14; G. P. Copenhaver et al. Science, 286, 2468-2479, 1999). By combining the sizes of the centromeric regions previously estimated for chromosomes 4 and 5 and the sequence data reported for the A. thaliana genome, the total genome size of A. thaliana was estimated to be approximately 146.0 Mb.

We have determined the genome structure of the centromeric region of Arabidopsis thaliana chromosome 4 by sequence analysis of BAC clones obtained by genome walking, followed by construction of a physical map using DNA of a hypomethylated strain. The total size of the centromeric region, corresponding to the recombinant inbred (RI) markers between mi87 and mi167, was approximately 5.3 megabases (Mb). This value is over 3 Mb longer than that previously estimated by the Arabidopsis Genome Initiative (Nature, 408, 796-815, 2000). Although we could not cover the entire centromeric region by BAC clones because of the presence of highly repetitive sequences in the middle (2.7 Mb), the cloned regions spanning approximately 1 Mb at both sides of the gap were newly sequenced. These results together with the reported sequences in the adjacent regions suggest that the centromeric region is principally composed of a central domain of 2.7 Mb, consisting of mainly 180-bp repeats and Athila elements, and upper and lower flanking regions of 1.55 Mb and 1 Mb, respectively. The flanking regions were predominantly composed of various types of transposable elements, except for the upper end moiety in which a large 5S rDNA array (0.65 Mb) and central domain-like sequence are present. Such an organization is essentially identical to the centromeric region of chromosome 5 reported previously (N. Kumekawa et al., DNA Res., 7, 315-321, 2000).

The sequence determination of the entire genome of the Synechocystis sp. strain PCC6803 was completed. The total length of the genome finally confirmed was 3,573,470 bp, including the previously reported sequence of 1,003,450 bp from map position 64% to 92% of the genome. The entire sequence was assembled from the sequences of the physical map-based contigs of cosmid clones and of lambda clones and long PCR products which were used for gap-filling. The accuracy of the sequence was guaranteed by analysis of both strands of DNA through the entire genome. The authenticity of the assembled sequence was supported by restriction analysis of long PCR products, which were directly amplified from the genomic DNA using the assembled sequence data. To predict the potential protein-coding regions, analysis of open reading frames (ORFs), analysis by the GeneMark program and similarity search to databases were performed. As a result, a total of 3,168 potential protein genes were assigned on the genome, in which 145 (4.6%) were identical to reported genes and 1,257 (39.6%) and 340 (10.8%) showed similarity to reported and hypothetical genes, respectively. The remaining 1,426 (45.0%) had no apparent similarity to any genes in databases. Among the potential protein genes assigned, 128 were related to the genes participating in photosynthetic reactions. The sum of the sequences coding for potential protein genes occupies 87% of the genome length. By adding rRNA and tRNA genes, therefore, the genome has a very compact arrangement of protein- and RNA-coding regions. A notable feature on the gene organization of the genome was that 99 ORFs, which showed similarity to transposase genes and could be classified into 6 groups, were found spread all over the genome, and at least 26 of them appeared to remain intact. The result implies that rearrangement of the genome occurred frequently during and after establishment of this species.

We have determined the size of the centromeric region of Arabidopsis thaliana chromosome 5, which corresponds to the genetically defined centromere by Copenhaver et al. (Science, 286, 2468-2474, 1999) on the basis of restriction analysis. As a large clone gap was present in the previously constructed contig map of the centromeric region, the restriction map of this region was constructed using Asc I, Not I, Apa I and Pme I and genomic DNA from a hypomethylated strain. The size of the centromeric region finally estimated by combination with the sequence data of cloned regions at both sides was 4.35 megabases (Mb). This value is over 2 Mb longer than those estimated in our previous work and also by Copenhaver et al. Combing this centromeric region with the physical map previously constructed, the entire length of chromosome 5 becomes 31 Mb. Although the internal moiety of the centromeric region has not been sequenced yet because of extremely high repetition, the result of sequence analysis from both sides toward the inside strongly suggests that the centromeric region is composed of the central 2.9-Mb domain consisting of mainly 180-bp repeats and Athila retrotransposons and flanking regions containing various types of transposons. On the basis of these observations, a structural model for the centromeric region is discussed.

Previously, we have reported a fine physical map of Arabidopsis thaliana chromosome 5, except for the centromeric and telomeric regions, by ordering clones from YAC, P1, TAC, and BAC libraries of the genome consisting of the two contigs of upper arm and lower arm, 11.6 M bases and 14.2 M bases, respectively. Here, the remaining centromeric and telomeric regions of chromosome 5 are completely characterized by the ordering of clones and PCR amplifications. Chromosome 5 of Arabidopsis thaliana ecotype Columbia is about 28.4 M bases long. The centromeric region is estimated at about 2 M bases long between two 5S-rDNA clusters. The 180-bp repeat region mainly consists of blocks of 180-bp tandem family and various type retroelements dispersed over a 500-kb region. The telomeric regions of chromosome 5 are characterized by PCR cloning, sequencing and hybridization. The telomere repeats at both ends are about 2.5-kb long and interestingly, telomere-associated repeats (approximately 700 bp) are found near both ends of chromosome 5.