Molecular cloning, tissue distribution and nutritional regulation of a 6-fatty acyl desaturase-like enzyme in large yellow croaker (Larimichthys crocea)

In this study, the full-length cDNA of a 6-fatty acyl desaturase-like (6-Fad-like) enzyme was first cloned from large yellow croaker, Larmichthys crocea. The cDNA was 2049bp, with a 107bp 5-UTR, a 604bp 3-UTR, and an ORF of 1338 that specified a protein of 445 amino acids. It contains two membrane-spanning domains, three histidine-rich regions and a cytochrome b(5) domain, which all align perfectly with the same domains located in other recently identified vertebrate 5 and 6 desaturases. Sequence comparison showed that the predicted protein revealed a high percentage identity (>70%) with 6 desaturases from other marine fish species. Tissue distribution analysis revealed that 6-Fad-like was expressed at highest level in brain, much lower level in liver and gill, and lowest level in spleen, heart, stomach and intestine. The hepatic mRNA levels of 6-Fad-like showed statistically negative relationship relative to increasing dietary n-3LC-PUFA and DHA/EPA (P<0.05, R>0.6). Transcription of 6-Fad-like in liver of fish fed diets with low and moderate level of n-3LC-PUFA (0.15%, 0.60% and 0.98% dry diet) was significantly increased by more than 20-fold than that with higher n-3LC-PUFA (1.37%, 1.79% and 2.25% dry diet; P<0.05). The transcriptional levels of 6-Fad-like in the liver of fish fed diets with the ratio of 0.61, 1.54 and 2.17 (DHA/EPA) were significantly up-regulated by about 46-fold, 4.8-fold, and 1.2-fold compared with that in the control group (DHA/EPA=3.88) respectively (P<0.05). This could contribute to better understanding the process of n-3LC-PUFA biosynthesis in this fish species.