Planta (Berl.) 122, 27--36 (1975) 9 by Springer-Verlag 1975
Ammonium Repression of Nitrate Reductase Formation in Lemna minor L. T. O. Orebamjo* and G. R. Stewart** Department of Botany, The University, )Ianchester M13 9PL, U.K. Received 17 September; accepted 6 November, 1974 Summary. The presence of ammonium is shown to inhibit the nitrate-promoted formation of nitrate reductase in Lemna minor L. The ammonium inhibition does not result from an inhibition of nitrate accumulation. The kinetics of the ammonium inhibition suggest it is not a direct effect of ammonium. The inhibition could result from the build up of a product of ammonium assimilation or an ammonium induced regulatory protein. It is suggested that both the nitrate stimulation and the 'ammonium' inhibition of nitrate reductase formation can be accounted for by a transcriptional control system.
Introduction I t has been suggested that in L e m n a m i n o r nitrate reductase formation is regulated by a dual control system in which nitrate acts as an inducer while ammonium and other products of nitrate assimilation act as end product repressors (Stewart, 1972a; Orebamjo and Stewart, 1974). Repression or inhibition of nitrate reductase formation is a widespread phenomenon having been reported in m a n y species of fungi (Morton, 1956; Kinsky, 1961 ; Cove, 1966; Sims, Folkes and Bussey, 1968 ; Lewis and Fincham, 1970) and in several species of algae (Morris and Syrett, 1963; Losada, Paneque, Aparicio, Vega, Cardenas and Herrera, 1970; Rigano, 1971 ; Thacker and Syrett, 1972). I n higher plants however ammonium repression does not appear to be a common phenomenon, although it has recently been reported in species other t h a n those of the Lemnaceae (Smith and Thompson, 1971; Stewart, Lee, Orebamjo and Havill, 1974). The precise mechanisms through which nitrate and ammonium exert their effects has not been uaequivocally determined for any system although enzyme induction appears to require both R N A and protein synthesis (Afridi and Hewitt, 1965; Ingle, 1966; Stewart, 1968; Lewis and Fincham, 1971; Subramanian and Sorger, 1972a, b; Oaks, Wallace and Stevens, 1972). With few exceptions (see for example Cove, 1970 ; Sorger and Davies, 1973) the general interpretation of these results is almost inevitably along the lines of the Jacob-Monod model (1961); in which nitrate is assumed to function as a co-inducer and ammonium as a corepresser. However the level at which they act is by no means well established and conflicting results have been obtained using different systems. I n N e u r o s p o r a crassa Sorger and D~vies (1973) suggest that nitrate promotes the translation of nitrate reductase ml~NA while ammonium inhibits m R N A transcription. I n con* Present address: The School of Biological Sciences, The University of Lagos, Lagos, Nigeria ** Adress for reprint requests
28
T.O. Orebamjo and G. R. Stewart A
Io
/
4,9
A
/
3.0
8
o
E
.
Ammonium Repression of Nitrate Reductase Formation in Lemna minor L. T. O. Orebamjo* and G. R. Stewart** Department of Botany, The University, )Ianchester M13 9PL, U.K. Received 17 September; accepted 6 November, 1974 Summary. The presence of ammonium is shown to inhibit the nitrate-promoted formation of nitrate reductase in Lemna minor L. The ammonium inhibition does not result from an inhibition of nitrate accumulation. The kinetics of the ammonium inhibition suggest it is not a direct effect of ammonium. The inhibition could result from the build up of a product of ammonium assimilation or an ammonium induced regulatory protein. It is suggested that both the nitrate stimulation and the 'ammonium' inhibition of nitrate reductase formation can be accounted for by a transcriptional control system.
Introduction I t has been suggested that in L e m n a m i n o r nitrate reductase formation is regulated by a dual control system in which nitrate acts as an inducer while ammonium and other products of nitrate assimilation act as end product repressors (Stewart, 1972a; Orebamjo and Stewart, 1974). Repression or inhibition of nitrate reductase formation is a widespread phenomenon having been reported in m a n y species of fungi (Morton, 1956; Kinsky, 1961 ; Cove, 1966; Sims, Folkes and Bussey, 1968 ; Lewis and Fincham, 1970) and in several species of algae (Morris and Syrett, 1963; Losada, Paneque, Aparicio, Vega, Cardenas and Herrera, 1970; Rigano, 1971 ; Thacker and Syrett, 1972). I n higher plants however ammonium repression does not appear to be a common phenomenon, although it has recently been reported in species other t h a n those of the Lemnaceae (Smith and Thompson, 1971; Stewart, Lee, Orebamjo and Havill, 1974). The precise mechanisms through which nitrate and ammonium exert their effects has not been uaequivocally determined for any system although enzyme induction appears to require both R N A and protein synthesis (Afridi and Hewitt, 1965; Ingle, 1966; Stewart, 1968; Lewis and Fincham, 1971; Subramanian and Sorger, 1972a, b; Oaks, Wallace and Stevens, 1972). With few exceptions (see for example Cove, 1970 ; Sorger and Davies, 1973) the general interpretation of these results is almost inevitably along the lines of the Jacob-Monod model (1961); in which nitrate is assumed to function as a co-inducer and ammonium as a corepresser. However the level at which they act is by no means well established and conflicting results have been obtained using different systems. I n N e u r o s p o r a crassa Sorger and D~vies (1973) suggest that nitrate promotes the translation of nitrate reductase ml~NA while ammonium inhibits m R N A transcription. I n con* Present address: The School of Biological Sciences, The University of Lagos, Lagos, Nigeria ** Adress for reprint requests
28
T.O. Orebamjo and G. R. Stewart A
Io
/
4,9
A
/
3.0
8
o
E
.
