Abundance of an mRNA encoding a high mobility group DNA-binding protein is regulated by light and an endogenous rhythm

Abstract

A cDNA clone encoding an HMG1 protein from Pharbitis nil was characterized with regard to its sequence, genomic organization and regulation in response to photoperiodic treatments that control floral induction. The HMG1 cDNA contains an open reading frame of 432 nucleotides encoding a 144 amino acid protein of approximately 16 kDa. The predicted polypeptide has the characteristic conserved motifs of the HMG1 and HMG2 class of proteins including an N-terminal basic region, one of two HMG-box domains, and a polyacidic carboxy terminus. Within the HMG-box region, Pharbitis HMG1 deduced amino acid sequence shares 47%, 67% and 69% identity with its animal, maize, and soybean counterparts, respectively. Southern blot hybridization analysis suggests that HMG1 is a member of a multigene family. Analysis of mRNA abundance indicates that the HMG1 gene is expressed to higher levels in dark-grown tissue, such as roots, and at lower levels in light-grown tissue, such as cotyledons and stems. Following the transition to darkness, the levels of HMG1 mRNA in cotyledons were initially stable, however, after a lag time of 8 h or more, HMG1 mRNA increased in abundance to a peak level at 20 h. A second peak in mRNA levels was observed about 24 h later, indicating that the expression of the HMG1 gene is regulated by an endogenous circadian rhythm. Abundance of the HMG1 mRNA during a dark period was dramatically affected by brief light exposure (night break), a treatment which inhibits floral induction. These data indicate that the expression of HMG1 is regulated by both an endogenous rhythm and the light/dark cycle and are consistent with a role for HMG1 in maintaining patterns of circadian-regulated gene expression activated upon the transition from light to darkness.

Dark and Circadian Regulation of mRNA Accumulation in the Short-Day Plant Pharbitis nil

“…Transcript levels were not significantly affected by transfer to darkness, maintaining approximately the same levels of abundance during the first 12 to 16 h of darkness. However, after an initial lag, levels declined continuously in the dark and failed to show the second transitory peak with a circadian periodicity suggestive of a rhythmic pattern of expression as was observed for PN1, PN9, PN11 (Zheng et al, 1993), and Cab. Several other cDNA clones, including PN12, which encodes peptidyl-prolyl cis-trans isomerase (S.D.…”

“…However, the observed dark-induced increases in mRNA abundance were unique to these two cDNA clones, with the exception of one other cDNA clone, PN11. This latter clone also showed an increase in mRNA levels in darkness, but in the case of PN11, the first and second mRNA peaks occurred later than those of PN1 or PN9, at 20 h and 48 h, respectively (Zheng et al, 1993). Thus, maximum PN11 transcript levels did not occur until after 20 h of darkness, a pattern of expression clearly displaced in dark time from that of PN1 and PN9, whose mRNAs had already cycled up and down to their lowest levels by this time.…”

“…A similar response was observed for Cab expression. Several other clones, including PN11 and PN12, were also examined and either showed an opposite response to NB as in the case of PN11 (Zheng et al, 1993) or failed to show any change in transcript level as observed for PN12 (data not shown). Hybridization to the actin probe demonstrated equal reactivity and equal amounts of RNA in all samples, thus supporting the conclusion that the observed differences in transcript levels were the result of NB treatment.…”

“…For example, cDNA clones representing a number of nuclear-encoded genes involved in photosynthesis were identified, including Cab, RbcS, Rubisco activase, PsaD, PsaE, PsaG, PsaH, and PsbR. We also identified sequences not related to photosynthesis, including those encoding Ser carboxypeptidase, Gln synthase, high mobility group 1 DNA-binding protein (Zheng et al, 1993), and peptidyl-prolyl cis-trans isomerase. In other cases, the nucleotide sequence did not correspond to a previously characterized gene.…”

Plant chromosomal high mobility group (HMG) proteins

“…Underlined within the HMG-box are three potential a-helical regions (I, II, III), as predicted by comparison with the rat HMG1 Bdomain structure determined by NMR spectroscopy (Weir et al, 1993). unpublished), although there is one report describing expression of an HMG-box protein regulated by light and an endogenous rhythm in Japanese morning glory (Zheng et al, 1993). Expression regulated by an endogenous circadian rhythm seems at odds with the rather high stability observed for mammalian HMG1/2 proteins (Johns, 1982); the in vivo half-life of the HMGa protein has been shown to be similarly high (~65 h), using pulse-chase experiments in a maize cell suspension culture (Grasser et al, 1994a).…”