No abstract

No abstract

We have isolated and characterized auxotrophic mutants of Trichosporon adeninovorans, strain PAR-4 to get genetic markers that cover the entire nuclear genome of this thermotolerant yeast of technological interest. The nitrosoguanidine mutagenesis yielded mutants at a high frequency. We detected a broad spectrum of auxotrophic phenotypes in the random mutant samples. Obviously, strain PAR-4 is a haploid or hyperhaploid yeast. In correspondence we determined a low DNA content per cell. In contrast to NG1), UV light was an inefficient mutagen. UV survival curves were without the typical shoulder indicating suppression of repair of UV-induced lethal lesions. Thus, the response of PAR-4 to UV was different from those of Saccharomyces cerevisiae and other yeasts.

Hybrids produced by protoplast fusion between auxotrophic mutants of thesame strain contained the parental markers heterozygously, as indicated by induced mitotic segregation. Even within the same fusion combination the hybrids differed in their mitotic stability and their induced mitotic segregation pattern. The occurrence of aneuploidsvas proved. The ploidy of the hybrids did not exceed the diploid level in any case.

The kinetics, efficiency, and specificity of induction of forward mutations to auxotrophy by ultraviolet light (UV) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined in stationary phase cells of Rhodosporidium (Rhodotorula) wild strain Rg1. In comparison to the spontaneous level the frequency of auxotrophic mutants was increased more than 1000 times by both mutagens, however, the mutagenic efficiency of MNNG was higher than that of UV. We found that the forward mutation rate is a linear function of the applicated UV and MNNG doses in the range to 600 J m-2 or 25 mM X min, respectively. The 35 studied biosynthetic pathways to amino acids, purines, pyrimidines, and vitamins are genetically blocked at different frequencies, but there is not any significant difference between UV and MNNG induced frequencies of mutants with a specific requirement. However, in difference to the approximately equal distribution of the MNNG-induced nic mutants among the genetic blocks of the tryptophan-nicotinamide pathway, UV-induced nic mutants occurred with a higher frequency in the genes of the tryptophan pyrrolase and the 3-hydroxykynureninase than in the genes of the other enzymes of the pathway.