PubMed · 6653950
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1983. Orphan biologic licensed.. https://pubmed.ncbi.nlm.nih.gov/6653950/
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Heme, an organometallic tetrapyrrole, is widely engaged in oxygen transport, electron delivery, enzymatic reactions, and signal transduction. In plants, it is also involved in photomorphogenesis and photosynthesis. HEME OXYGENASE 1 (HO1) initiates the first committed step in heme catabolism, and it has generally been thought that this reaction takes place in chloroplasts. Here, we show that HO1 in both Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) has 2 transcription start sites (TSSs), producing long (HO1L) and short (HO1S) transcripts. Their products localize to the chloroplast and the cytosol, respectively. During early development or de-etiolation, the HO1L/HO1S ratio gradually increases. Light perception via phytochromes (Phys) and cryptochromes elevates the HO1L/HO1S ratio in the whole seedling through the functions of ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG and through the suppression of DE-ETIOLATED 1, CONSTITUTIVE PHOTOMORPHOGENESIS 1, and PHYTOCHROME INTERACTING FACTORs. HO1L introduction complements the HO1-deficient mutant; surprisingly, HO1S expression also restores the short hypocotyl phenotype and high pigment content and helps the mutant recover from the genomes uncoupled (gun) phenotype. This indicates the assembly of functional Phys within these lines. Furthermore, our findings support the hypothesis that a mobile heme signal is involved in retrograde signaling from the chloroplast. Altogether, our work clarifies the molecular mechanism of HO1 TSS regulation and highlights the presence of a cytosolic bypass for heme catabolism in plant cells.
The normal mode analysis of human hemoglobin showed the presence in the deoxy T-state of one main preferential direction that brings the structure close to the R-state, with a low-energy variation, while in the oxy R-state there are several modes that point towards the T-state, but with higher energy variations and less contribution to the transition. The displacement along a combination of normal modes, followed by energy minimization, starting from the R-state, did not allow one to obtain a structure significantly different from that of R, showing that the fully oxygenated hemoglobin is trapped in a deep and narrow potential energy minimum. On the contrary, starting from the deoxy T-state, the displacement along a combination of normal modes, followed by energy minimization, yielded an intermediate structure, that we designate Tmin(d1), which is closer to R; the normal modes of Tmin(d1) indicated that the potential energy minimum in the vicinity of this structure is as narrow as that of R but less deep. The procedure of displacement along the modes, followed by energy minimization, was applied to Tmin(d1), yielding Tmin(d2); then the procedure was repeated, yielding the intermediate structures Tmin(d3) and Tmin(d4). The structures Tmin(d2), Tmin(d3) and Tmin(d4) are not significantly different from each other, indicating that they are trapped in a narrow, deep energy minimum. This procedure revealed the existence of at least two intermediate sets of structures between T and R: the first one, Tmin(d1), is different from the T and R structures, while the second set, Tmin(d2), Tmin(d3) and Tmin(d4), is quaternary R-like and tertiary T-like, where the contacts at the interfaces alpha1 beta1 and alpha1 beta2 are R-like, and the alpha and beta heme environments are still T-like.
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