NITRATE TRANSPORT IN THE CYANOBACTERIUM ANACYSTIS-NIDULANS

A significant progress in the knowledge of different aspects of nitrat e transport in the unicellular cyanobacterium Anacystis (Synechococcus ) has been achieved in the last few years. The main contributions of o ur group are summarized in this article and discussed in relation to o ther information available. Endergonic accumulation of nitrate into th e cells, indicative of the operation of an active nitrate transport sy stem, has been experimentally substantiated and methods established to evaluate and analyze the activity of the system. Nitrate transport ac tivity is sensitive to regulation exerted by products of both ammonium and CO2 assimilation, thus providing evidence that photosynthetic nit rate assimilation in cyanobacteria is primarily controlled at the leve l of substrate supply to the cell. The expression of nitrate transport was also shown to be under nitrogen control, being repressed when amm onium is used as the nitrogen source. A 47-kDa polypeptide, which is a major plasma membrane component in nitrate-grown cells but is virtual ly absent in ammonium-grown cells, was identified as an essential comp onent of the nitrate transporter. More recently, evidence of a strict Na+-dependence of active nitrate transport has been obtained, Delta mu (Na+) appearing as the driving force of a sodium-nitrate symport syste m. Kinetic studies indicate also that the nitrate transporter may tran sport nitrite into the cell.