Plant Molecular Biology 41: 89–104, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 89 Expression of photosynthesis genes in relation to nitrogen fixation in the diazotrophic filamentous nonheterocystous cyanobacterium Trichodesmium sp. IMS 101 Yi-Bu Chen 1 , Benny Dominic, Sabino Zani, Mark T. Mellon and Jonathan P. Zehr 2,∗ Biology Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590, USA; current addresses: 1 Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA; 2 Ocean Sciences Department, University of California, 1156 High Street, Santa Cruz, CA 95064, USA ( ∗ author for correspondence) Received 9 December 1998; accepted in revised form 2 July 1999 Key words: cyanobacteria, circadian rhythm, psaA, psbA, Trichodesmium sp. Abstract The daily cycle of nitrogenase expression in the marine filamentous nonheterocystous cyanobacterium Tri- chodesmium spp. is controlled by a circadian rhythm. We evaluated the rhythm of two key photosynthesis genes, psbA of photosystem II and psaA of photosystem I, in Trichodesmium sp. IMS 101 using the 3 criteria for an endogenous rhythm. The transcript abundance of psbA and psaA transcripts oscillated with a period of ca. 24 h under a 12 h light/12 h dark regime. At 24 ◦ C and 28 ◦ C the cyclic pattern of transcript abundance was maintained for at least 58 h under constant light conditions, whereas the periods were about 24 h at 24 ◦ C, and 26–30 h at the higher temperature. The cycles of psbA and psaA gene expression were entrained using light-dark cues. Transcription of nifHDK was initiated prior to the light period, followed by psbA and finally psaA. There was a 90 ◦ (6 h) phase difference between the net accumulation of nifHDK and psbA transcripts, as well as between that of psbA and psaA transcripts. Results of inhibitor experiments indicated that psbA and psaA transcription was regulated differently by initiation and degradation during the light period. Short-term changes of light conditions resulted in significant effects on psbA transcription and nitrogenase activity, but had less of an effect on psaA and nifHDK transcription. Introduction Trichodesmium spp. are diazotrophic filamentous non- heterocystous marine cyanobacteria. Trichodesmium contributes significantly to the annual input of new ni- trogen to the nutrient-limited surface waters of tropical and subtropical oceans all over the world (Carpenter, 1983; Gallon et al., 1996; Capone et al., 1997; Zehr et al., 1998). Trichodesmium fixes nitrogen exclu- sively during the light when oxygenic photosynthesis is concurrent, without any known mechanism to either temporally or spatially separate oxygenic photosyn- The nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession numbers AF107784 (psbA) and AF107783 (psaA). thesis from oxygen-sensitive nitrogen fixation. How Trichodesmium protects nitrogenase from inactivation by oxygen is a perplexing and fascinating question. In the past, relatively few studies have focused on photosynthesis by Trichodesmium. Based on parallel 14 CO 2 and N 2 fixation measurements, it was sug- gested that the patterns of photosynthetic CO 2 fixation in both cultured and natural Trichodesmium popula- tions were similar and corresponded over time to the patterns of N 2 fixation (Paerl, 1994). Microautora- diographic data from the same study also provided some evidence of a possible spatial separation be- tween carbon and nitrogen fixation (Paerl, 1994). Results of studies using immunohistochemistry cou- pled with light and transmission electron microscopy