Identification of S2-T A63: a cDNA fragment corresponding to a gene differentially expressed in a Cr-tolerant strain of the unicellular green alga Scenedesmus acutus.

The gene expression of the wild type (S2-N) and a Cr-tolerant strain (S2-T) of the unicellular green alga Scenedesmus acutus has been compared in order to get more insight on their different chromium sensitivity. The RNA of the two strains was extracted after 4 days of culture in standard medium without chromium and analyzed by means of RNA differential display. The two strains showed differential gene transcription even in the absence of the heavy metal and six putatively differential amplicons were evidenced in the Cr-tolerant strain. Among the isolated amplicons, S2-T A63 was much more pronouncedly transcribed in the tolerant than in the wild type strain and was further characterized. S2-T A63 corresponding gene is present with the same copy number in the wild type and tolerant genomes and corresponds to an mRNA of about 2000 nt. The corresponding transcript is overexpressed in the Cr-tolerant strain after a 4-day culture and is not up-regulated by chromium exposure. The S2-T A63 sequence, obtained up to now, does not show significant homologies with any known gene. However, the analysis of the putative translation product reveals the presence of an interrupted fasciclin domain. This extracellular domain has been found in proteins from mammals, insects, echinoderms, plants, yeast and bacteria and is usually involved in cell adhesion. This finding suggests that the product for the S2-T A63 translation has an extracellular collocation, maybe as surface or secreted protein involved in external chromium detoxification.