RESUMEN
Three Delta6 desaturase-defective mutants, designated YB214, HR95, and ST66, were newly isolated from Mortierella alpina 1S-4, after treating wild-type spores with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). These three mutants and Mut49, isolated previously, are capable of accumulating 5,11,14-cis-eicosatrienoic acid (20:3Delta5). Two functional Delta6 desaturases (Delta6I and Delta6II) were found to exist in M. alpina 1S-4. The mutation sites on the Delta6I gene in the Delta6 desaturase-defective mutants were identified. The mutations each resulted in an amino acid replacement (W314Stop, T375K, and G390D) in Delta6I from ST66, HR95, and YB214 respectively, and uncorrected transcription of the Delta6I gene in Mut49 was caused by disappearance of the GT-terminal of the second intron, resulting in low Delta6 desaturation activity in these mutants. On the other hand, there was no mutation site on the Delta6II genes of the mutants.