The ribosomal DNA of the Zygomycete Mucor miehei

The ribosomal DNA from the Zygomycete Mucor miehei has been characterised. The complete rDNA unit was cloned by heterologous PCR using primers whose s equence matched conserved regions of the rDNA from related fungal species. The sequence of the overlapping PCR products revealed the existence of a re peated unit of 9574 bp. The genes encoding the different rRNA species were identified by their homology to the corresponding sequences from other fung i. We estimate that the rDNA unit is present in the genome of M. miehei in about 100 copies. This estimation was made by comparing the intensity of it s hybridisation signal in a Southern blot with that of the mmp gene coding for aspartyl protease, which was assumed to be contained in single copy. Th e size and structure of the M, miehei rDNA unit was similar to that of othe r fungi. The genes encoding the 25S, 18S and 5.8S RNAs are closely linked w ithin the repeated unit which also contains the 5S gene. This latter gene a ppears to be transcribed in the opposite direction. The 25S, 18S and 5.8S g enes showed 70-80% homology to the corresponding genes from other fungi, wh ereas the degree of homology for the 5S gene was much lower. The highest ho mology (about 80%) corresponded to the few available sequences from other M ucor species. Homology to genes from other Zygomycota was no higher than th at observed for genes from the Ascomycota or Basidiomycota fungi.