However, previous reports have indicated that carotenogenesis may be regulated by some type of feedback mechanism, by which the relative proportion of astaxanthin regulates the total amount of carotenoids synthesized . Given our observations, the feedback mechanism mediated by astaxanthin in the carotenoid biosynthesis pathway may involve regulatory mechanisms at the transcriptional level, and the presence and composition of pigments may affect the transcriptional response triggered by the addition of glucose to the medium. To test this hypothesis, we performed glucose addition experiments using homozygous mutant strains that are incapable of synthesizing astaxanthin.
The strains used were T-YBHH2 (crtYB – ), T-I21H1H (crtI -) and T-SHH2 (crtS – ), as described in a previous work . First, we Z-VAD-FMK molecular weight determined that the response of grg2 and PDC expression to glucose was similar in all of the strains studied and Selleck MCC-950 did not depend on the synthesis of selleckchem pigments (data not shown). In contrast, different results were observed for the mutants of the carotenogenesis pathway genes. For the crtYB gene, the 6-fold repression of mature messenger observed in the wild-type strain in response to glucose completely disappeared and was replaced by a slight induction in both, the mutant that accumulates phytoene (crtI -) and
the mutant that accumulates β-carotene (crtS – ) (Figure 5a). However, the levels of alternative transcripts in both mutant strains exhibited a glucose-mediated decrease that was less than the one observed in the wild-type strain (Figure 5b). A similar phenomenon was observed for the crtI gene; both, the mutant incapable of synthesizing carotenoids (crtYB aminophylline – ) and the mutant that accumulates β-carotene (crtS – ) showed a complete lack of glucose repression of the mature transcript (Figure 5c) and a very diminished response of the alternative transcript levels (Figure 5d). Finally, in the case of the crtS gene, the slight repression by glucose observed
in the wild-type strain was replaced by a slight induction in the crtYB – and crtI – mutants (Figure 5e). These results indicate that the expression of the crtYB, crtI and crtS genes in response to the addition of glucose depends, at least in part, on the normal synthesis of astaxanthin or on the presence of this compound in the cell. Figure 5 Effect of glucose on the expression of carotenogenesis genes in mutant strains incapable of synthesizing astaxanthin. Strains: T-YBHH2 (crtYB-/-, white inverted triangle), T-I21H1H (crtI-/-, black square) and T-SHH2 (crtS-/-, white diamond). Levels of mRNA for mature crtYB (a), alternative crtYB (b), mature crtI (c), alternative crtI (d) and crtS (e) in glucose-treated cultures (20 g/l) were determined for each strain relative to the control.