ORGANIZATION OF THE qa GENE CLUSTER IN NEUROSPORA CRASSA: DIRECTION OF TRANSCRIPTION OF THE qa-3 GENE

[摘要] In Neurospora crassa, the enzyme quinate (shikimate) dehydrogenase catalyzes the first reaction in the inducible quinic acid catabolic pathway and is encoded in the qa-3 gene of the qa cluster. In this cluster, the order of genes has been established as qa-I qa-3 qa-4 qa-2 . Amino-terminal sequences have been determined for purified quinate dehydrogenase from wild type and from UV-induced revertants in two different qa-3 mutants. These two mutants (M16 and M45) map at opposite ends of the qa-3 locus. In addition, mapping data (CASE et al. 1978) indicate that the end of the qa-3 gene specified by M45 is closer to the adjacent qa-1 gene than is the end specified by the MI6 mutant site. In one of the revertants (R46 from qa-3 mutant M45), the amino-terminal sequence for the first ten amino acids is identical to that of wild type. The other revertant (R1 from qa-3 mutant M16) differs from wild type at the amino-terminal end by a single altered residue at position three in the sequence. The observed change involves the substitution of an isoleucine in M16-Rl for a proline in wild type. This substitution requires a two-nucleotide change in the corresponding wild-type codon.——The combined genetic and biochemical data indicate that the qa-3 mutants M16 and M45 carry amino acid substitutions near the amino-terminal and carboxyl-terminal ends of the quinate dehydrogenase enzyme, respectively. On this basis we conclude that transcription of the qa-3 gene proceeds from the end specified by the MI6 mutant site in the direction of the qa-1 gene. It appears probable that transcription is initiated from a promoter site within the qa cluster, possibly immediately adjacent to the qa-3 gene.

[学科分类] 医学（综合）