Organisation of the complex locus trp1 in the fission yeast Schizosaccharomyces pombe.

65 trp1(-) alleles of Schizosaccharomyces pombe have been analysed for their interallelic complementation pattern, suppressibility by nonsense suppressors and position of the corresponding mutation site on an intragenic map of the trp1 locus. In addition to the three complementation classes previously described (Schweingruher and Dietrich 1973) as defective in phosphoribosylanthranilate isomerase (trp1A), indole glycerolphosphate synthetase (trp1B) and anthranilate synthetase (trp1C), two new complementation classes, trp1BC and trp1ABC, were found. The former is represented by a single allele which can only complement trp1A mutants. The latter is represented by six alleles which fail to complement tester mutants of the trp1A, trp1B or trp1C class. Classes trp1A, trp1B and trp1C correspond to mutations in three nonoverlapping regions mapping in the order trp1A, trp1B and trp1C All the alleles of the trp1BC and trp1ABC classes correspond to mutations in the trp1C region. Nonsense alleles of the opal (UGA) or ochre (UAA) type were found in the trp1A (2 alleles out of 17) and trp1ABC (5 alleles out of 6) classes only. These data indicate that the trp1 locus is transcribed as a single messenger RNA with transcription starting from the trp1C region. This messenger is probably translated in a single, multifunctional polypeptide, or at most in two polypeptides coded for by the trp1B-trp1C and by the trp1A regions. In addition, the polar effect of nonsense mutations in the trp1C region is cancelled by rare spontaneous mutations occuring at or very near the trp1 locus, which may act by creating an internal single for the initiation of transcription and/or translation.