Light-induced expression of fatty acid desaturase genes

Abstract

In cyanobacterial cells, fatty acid desaturation is one of the crucial steps in the acclimation processes to low-temperature conditions. The expression of all the four acyl lipid desaturase genes of Synechocystis PCC 6803 was studied as a function of temperature and separately as a function of light. We used cells grown at 25 degreesC in light-activated heterotrophic growth conditions. In these cells, the production of alpha-linolenic acid and 18:4 fatty acids was negligible and the synthesis of gamma-linolenic acid was remarkably suppressed compared with those of the cells grown photoautotrophically. The cells grown in the light in the presence of glucose showed no difference in fatty acid composition compared with cells grown photoautotrophically. The level of desC mRNA for delta9 desaturase was not affected by either the temperature or the light. It was constitutively expressed at 25 degreesC with and without illumination. The level of desB transcripts was negligible in the dark-grown cells and was enhanced about 10-fold by exposure of the cells to light. The maximum level of expression occurred within 15 min. The level of desA and desD mRNAs was higher in dark-grown cells than that of desB mRNA for omega3 desaturase. However, the induction of both desA and desD mRNAs for delta12 and delta6 desaturases, respectively, was enhanced by light about 10-fold. Rifampicin, chloramphenicol, and 3-(3, 4-dichlorophenyl)-1,1-dimethylurea completely blocked the induction of the expression of desA, desB, and desD. Consequently, we suggest the regulatory role of light via photosynthetic processes in the induction of the expression of acyl lipid desaturases.

Figure 1

Expression level of Synechocystis PCC 6803 desaturase genes is regulated differentially by light. Cells were grown heterotrophically in the dark at 25°C, then transferred to light, incubated for 120 min, and subsequently transferred back to dark for an additional 120 min. The first sample (0 min) was taken immediately after transferring the culture from dark to light and additional samples were collected as indicated. Each lane contains 10 μg of total RNA. (A) mRNA levels of the four desaturase genes were determined by Northern blot analyses. (B) Relative changes of mRNA levels are shown in percentage and were obtained after quantification with a PhosphorImager.

Figure 2

Expression level of Synechocystis PCC 6803 desaturase genes is regulated differentially by changes in the ambient temperature. Cells were grown autotrophically at 35°C, then transferred to 25°C for 120 min, and subsequently transferred back to 35°C for an additional 120 min. The first sample (0 min) was taken immediately after transferring the culture from 35°C to 25°C, and additional samples were collected as indicated. Each lane contains 10 μg of total RNA. (A) mRNA levels of the four desaturase genes were determined by Northern blot analysis. (B) Relative changes of mRNA levels are shown in percentage and were obtained after quantification with a PhosphoImager.

Figure 3

Effect of temperature down-shift in the dark on the mRNA level of the desaturase genes in Synechocystis PCC6803 cells as demonstrated by Northern blot analyses. Cells were grown photoheterotrophically at 35°C under light and then incubated in the dark at 25°C. The first sample (0 min) was taken immediately after transferring the culture from 35°C to 25°C and additional samples were collected as indicated. Each lane contains 10 μg of total RNA.

Figure 4

Effects of transcription (Rif), translation (Cm), and photosynthesis (DCMU) inhibitors on the expression of desaturase genes. mRNA levels were determined by Northern blot analysis. Each lane contains 5 μg of total RNA. Cells were grown heterotrophically at 25°C in dark and then transferred to light in the absence (control) or presence (+) of the inhibitors. When DCMU was used, the cells were preincubated for 1 min in darkness. The first sample (0 min) was taken immediately before transferring the cultures to light, and additional samples were collected as indicated. Rif, rifampicin, 200 μg/ml; Cm, chloramphenicol, 40 μg/ml; DCMU, 10 μM.

Figure 5

Scheme for the expression of fatty acid desaturase genes in Synechocystis PCC 6803 cells at 25°C in the light (open arrows) and in the dark (solid arrows). The difference in width of the arrows represents the activity of the expression of the individual genes.