Biomedical subjects

Takamasa Suzuki

Publications and source records attributed to Takamasa Suzuki.

3 recordsLinked to original sources

PubMed2026

Arabidopsis TITAN-LIKE is required for U12-type intron splicing, especially of AT-AC subtypes.

Many eukaryotes possess two types of spliceosomes: the U2-dependent and U12-dependent spliceosomes. The U2-dependent spliceosome processes >99% of all introns, whereas the U12-dependent spliceosome acts on only ~0.3% of introns, one-third of which start with AT and end with AC, with the remainder having GT-AG termini. How the U12-dependent spliceosome splices two types of introns with different terminal sequences remains poorly understood. Human centrosomal AT-AC splicing factor (CENATAC) is a subunit of the U12-dependent spliceosome that is particularly required for the splicing of the AT-AC subtype. The Arabidopsis genome contains a single homolog, TITAN-LIKE (TTL), but its function in splicing remains unknown. Here, we generated ttl mutants and isolated two viable alleles, of which we analyzed one, designated ttl-142, to investigate TTL's function in splicing. ttl-142 carries a 42-nucleotide deletion that removes 14 amino acid residues from the predicted protein, and homozygous mutants exhibit morphological abnormalities. Most U12-dependent introns were less efficiently spliced in ttl-142 than in the wild type, with the splicing of AT-AC introns particularly suppressed. Splicing suppression in ttl-142 was more extensive than in a drol1 (defective repression of the OLE3:LUC1) mutant, which carries a mutation in a gene specifically required for AT-AC intron splicing. Conversely, fewer genes showed altered expression levels in ttl-142 than in drol1, and most differentially expressed genes differed between the two mutants. These results suggest that the phenotypes of ttl-142 and drol1 mutants may reflect the impairment of distinct spliceosomal functions.

Arabidopsis↗

PubMed2025

Companion cells with high florigen production express other small proteins and reveal a nitrogen-sensitive FT repressor.

The precise onset of flowering is crucial to ensure successful plant reproduction. The gene FLOWERING LOCUS T (FT) encodes florigen, a mobile signal produced in leaves that initiates flowering at the shoot apical meristem. In response to seasonal changes, FT is induced in phloem companion cells located in distal leaf regions. Thus far, a detailed molecular characterization of the FT-expressing cells has been lacking. Here, we used bulk nuclei RNA-seq and single nuclei RNA (snRNA)-seq to investigate gene expression in FT-expressing cells and other phloem companion cells. Our bulk nuclei RNA-seq demonstrated that FT-expressing cells in cotyledons and true leaves showed differences especially in FT repressor genes. Within the true leaves, our snRNA-seq analysis revealed that companion cells with high FT expression form a unique cluster in which many genes involved in ATP biosynthesis are highly upregulated. The cluster also expresses other genes encoding small proteins, including the flowering and stem growth inducer FPF1-LIKE PROTEIN 1 (FLP1) and the anti-florigen BROTHER OF FT AND TFL1 (BFT). In addition, we found that the promoters of FT and the genes co-expressed with FT in the cluster were enriched for the consensus binding motifs of NITRATE-INDUCIBLE GARP-TYPE TRANSCRIPTIONAL REPRESSOR 1 (NIGT1). Overexpression of the paralogous NIGT1.2 and NIGT1.4 repressed FT expression and significantly delayed flowering under nitrogen-rich conditions, consistent with NIGT1s acting as nitrogen-dependent FT repressors. Taken together, our results demonstrate that major FT-expressing cells show a distinct expression profile that suggests that these cells may produce multiple systemic signals to regulate plant growth and development.

BROTHER OF FT AND TFL1↗

PubMed2025

DROL1/DIB1 determines U5 snRNP specificity for intron terminal dinucleotide in Arabidopsis.

Most introns contain GT-AG terminal dinucleotides; although some eukaryotes have introns with AT-AC termini whose splicing is impaired in the Arabidopsis defective repression of OLE3::LUC 1 (drol1) mutant. We identified seven drol1 suppressors across four loci, all encoding subunits of the U5 snRNP. Although AT-AC splicing was partially restored in these suppressors, their phenotypes were almost completely rescued. Artificial introns with either GT-AG or AT-AC termini showed a splicing preference for GT-AG termini in drol1 and its suppressors. These results suggest that AT-AC introns are spliced by a GT-AG specific spliceosome in the suppressors, with DROL1 influencing U5 snRNP specificity at the 5' splice site. We propose that mRNAs retaining unspliced AT-AC introns impair translation and produce nuclear signals that contribute to the complex phenotypes seen in drol1.

Arabidopsis↗