Transcriptional regulation of CDP1 and CYG56 is required for proper NH4+ sensing in Chlamydomonas

The assimilation of inorganic nitrogen is an essential process for all plant-like organisms. In the presence of ammonium and nitrate as nitrogen sources, Chlamydomonas reinhardtii preferentially assimilates ammonium and represses the nitrate assimilation pathway through an unknown mechanism that in part involves the guanylate cyclase CYG56. It is demonstrated that cells not only respond quantitatively to the NH4+ signal but are also able to sense a balance between both nitrogen sources. This quantitative response was altered in a collection of mutants that were partially insensitive to NH4+. In one of these mutants, reduced function of a gene named CDP1 encoding a cysteine domain-containing protein was genetically linked to NH4+ insensitivity. Alteration of CYG56 or CDP1 transcription was detected in several mutants, and combined down-regulation of both genes seemed to enhance the incapacity to sense NH4+ properly. These results suggest that transcriptional regulation of CYG56 and CDP1 are central and independent steps of the NH4+ signalling pathway.