Plant Physiol, March 2000, Vol. 122, pp. 731-736
Differential Expression of Photosynthesis and Nitrogen Fixation
Genes in the Cyanobacterium Plectonema boryanum
Hari S.
Misra* and
Rakesh
Tuli
Molecular Biology and Agriculture Division, Bhabha Atomic Research
Centre, Trombay, Mumbai 400 085, India (H.S.M.); and National Botanical
Research Institute, Rana Pratap Marg, Lucknow 226 001, India (R.T.)
The filamentous non-heterocystous
cyanobacterium Plectonema boryanum fixes dinitrogen at a
high rate during microaerobic growth in continuous illumination by
temporal separation of oxygen-evolving photosynthesis and
oxygen-sensitive dinitrogen fixation. The onset of nitrogen fixation is
preceded by a depression in photosynthesis that establishes a
sufficiently low level of dissolved oxygen in the growth medium. A
several-fold reduction in the level of transcripts coding for
phycocyanin (cpcBA) and the chlorophyll a
binding protein of photosystem II (psbC) and
psbA accompanied the depression in photosynthetic oxygen
evolution. Unlike most of the other organisms examined to date, in
P. boryanum, psbC and psbD do
not appear to be co-transcribed. The psbC transcripts were down-regulated several fold, while the psbD
transcript declined marginally during the nitrogen fixation phase. A
decrease in dissolved oxygen and a dramatic increase in the level of
nifH transcripts and the enzyme activity of nitrogenase
were characteristic of the nitrogen fixation phase. The level of
transcript for glnA, which encodes glutamine synthetase,
was not altered. Reciprocal regulation of gene expression was well
orchestrated with the alternating cycles of photosynthesis and nitrogen
fixation in P. boryanum.
*
Corresponding author; e-mail mbad{at}magnum.barct1.ernet.in;
fax 91-022-5505151.
© 2000 American Society of Plant Physiologists
