| Popis: |
Cyanobacteria form a very divergent group of unicellular or filamentous microorganisms, which constitute one of the largest subgroups of Gram-negative prokaryotes. They have very few nutrient requirements due to their capacity to carry out a plant-like, oxygen-evolving photosynthesis, so mainly carbon dioxide and nitrogen are needed in more than small amounts. Some strains can fix atmospheric nitrogen, but the majority relies on nitrate, nitrite or ammonia. Nitrate reduction in cyanobacteria is tightly coupled to photosynthesis by ferredoxin, which provides both nitrate reductase (NR) and nitrite reductase (NiR) with electrons. In contrast to higher plant NR, cyanobacterial NR consists of only a single polypeptide of about 80 kDa with one [Fe4S4] or two [Fe2S2] clusters and a molybdenum cofactor (Guerrero et al. 1981; Mikami and Ida 1984). Kuhlemeier (1984 a,b) desribed the construction of nitrate reduction mutants of Synechococcus PCC 7942 (formerly called Anacystis nidulans R2) by chemical mutagenesis with N-methyl-N’-nitro-N-nitrosoguanidine (MNNG) or transposon Tn901 mutagenesis (Tandeau de Marsac et al. 1982). Phenotypical characteristics of these mutants include: (a) slow growth and yellow color on nitrate medium, (b) complete restoration of growth and color after addition of nitrite to the medium, (c) no detectable nitrate reductase activity, even in the presence of 1 mM sodium molybdate in the medium. |
