Nucleic Acids Research, 2001, Vol. 29, No. 9 1835-1843
© 2001 Oxford University Press
An AU-box motif upstream of the SD sequence of light-dependent psbA transcripts confers mRNA instability in darkness in cyanobacteria
Laboratory of Molecular Genetics, School of Agriculture, Ibaraki University, Ami 3-21-1, Inashiki, Ibaraki 300-0332, Japan
The psbA2 gene of a unicellular cyanobacterium, Microcystis aeruginosa K-81, encodes a D1 protein homolog in the reaction center of photosynthetic Photosystem II. The expression of the psbA2 transcript has been shown to be light-dependent as assessed under light and dark (12/12 h) cycling conditions. We aligned the 5'-untranslated leader regions (UTRs) of psbAs from different photosynthetic organisms and identified a conserved sequence, UAAAUAAA or the ‘AU-box’, just upstream of the SD sequences. To clarify the role of 5'-upstream cis-elements containing the AU-box for light-dependent expression of psbA2, a series of deletion and point mutations in the region were introduced into the genome of heterologous cyanobacterium Synechococcus sp. strain PCC 7942, and psbA2 expression was examined. A clear pattern of light-dependent expression was observed in recombinant cyanobacteria carrying the K-81 psbA2 –38/+36 region (which includes the minimal promoter element and a light-dependent cis-element with the AU-box), +1 indicating the transcription start site. A constitutive pattern of expression, in which the transcripts remained almost stable under dark conditions, was obtained in cells harboring the –38/+14 region (the minimal element), indicating that the +14/+36 region with the AU-box is important for the observed light-dependent expression. Point mutations analyses within the AU-box also revealed that changes in number, direction and identity (as assayed by adenine/uridine nucleotide substitutions) influenced the light-dependent pattern of expression. The level of psbA2 transcripts increased markedly in CG- or deletion-box mutants in the dark, strongly indicating that the AU- (AT-) box acts as a negative cis-element. Furthermore, characterization of transcript accumulation in cells treated with rifampicin suggests that psbA2 5'-mRNA is unstable in the dark, supporting the view that the light-dependent expression is controlled at the post-transcriptional level. We discuss various mechanisms that may lead to altered mRNA stability such as the binding of factor(s) or ribosomes to the 5'-UTR and possible roles of the AU-box motif and the SD sequence.
* To whom correspondence should be addressed. Tel: +81 298 888652; Fax: +81 298 888653; Email: shirai{at}ipc.ibaraki.ac.jp The authors wish it to be know that, in their opinion, the first three authors should be regarded as joint First Authors
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  M. Asayama and S. Imamura Stringent promoter recognition and autoregulation by the group 3 {sigma}-factor SigF in the cyanobacterium Synechocystis sp. strain PCC 6803 Nucleic Acids Res., September 1, 2008; 36(16): 5297 - 5305. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Asayama, H. Kato, J. Shibato, M. Shirai, and T. Ohyama The curved DNA structure in the 5'-upstream region of the light-responsive genes: its universality, binding factor and function for cyanobacterial psbA transcription Nucleic Acids Res., November 1, 2002; 30(21): 4658 - 4666. [Abstract] [Full Text] [PDF]
|
|