M. Ohresser et al., EXPRESSION OF THE ARYLSULFATASE REPORTER GENE UNDER THE CONTROL OF THE NIT1 PROMOTER IN CHLAMYDOMONAS-REINHARDTII, Current genetics, 31(3), 1997, pp. 264-271
In Chlamydomonas reinhardtii, the expression of the nit1 gene encoding
nitrate reductase is dependent on the nature of the nitrogen source a
nd on other environmental factors. We have fused the nit1 promoter reg
ion to the arylsulphatase (ars) reporter gene lacking its own promoter
and introduced this chimeric construction (nit1/ars) into a wall-less
strain of C. reinhardtii. A new and sensitive method, based on the us
e of alpha-naphthylsulphate as a substrate and a diazonium salt as a c
hromogenic post-coupling agent, was developed to detect the activity o
f arylsulphatase (an enzyme which is almost completely secreted in the
culture medium) both in vitro and in agar plates. The transformants c
arrying nit1/ars did not express arylsulphatase when grown in ammonium
-sufficient medium but readily accumulated the enzyme in ammonium-free
medium either supplemented, or not supplemented, with nitrate or nitr
ite. The nit1/ars construct, however, was not expressed in the nit2 mu
tant lacking a specific transcription regulator controlling the expres
sion of nit1. These results, together with the observation that the tr
anscription of nit1/ars is initiated at the same sites as the nit1 end
ogenous gene, confirms the hypothesis that the regulation of nit1 expr
ession takes place mainly at the transcriptional level. The expression
of the ars gene from the nit1 promoter was high enough to allow direc
t measurements of arylsulphatase activities in pools of transformants
without prior isolation of nit1/ars clones. This original procedure ha
s permitted the analysis of the effects of nested deletions in the nit
1 promoter region on the expression of the reporter gene. The results
indicate that the -282 to -198 sequence is required for transcription
to occur and that the -751 to -282 region contains several elements me
diating nit1 expression.