Evolution of yeast ribosomal DNA: molecular cloning of the rDNA units of Kluyveromyces lactis and Hansenula wingei and their comparison with the rDNA units of other Saccharomycetoideae.

We have studied the evolution of the yeast ribosomal DNA unit to search for regions outside the rRNA genes that exhibit evolutionary constraints and therefore might be involved in control of ribosome biosynthesis. We have cloned one complete rDNA unit of Kluyveromyces lactis and Hansenula wingei and established the physical and genetic organisation of both units. Both species belong to the subfamily of the Saccharomycetoideae. The lengths of the rDNA units of K. lactis and H. wingei are 8.6 and 11.1 kb respectively, and both comprise the 5S rRNA gene in addition to the large rRNA operon. Sequence conservation was monitored by restriction enzyme mapping as well as heteroduplex analysis of the two cloned rDNA units with S. carlsbergensis rDNA. These analyses showed that, phylogenetically, K. lactis is closer to S. carlsbergensis than H. wingei. The non-transcribed spacers (NTS) of both K. lactis and H. wingei have diverged completely from S. carlsbergensis; moreover in H. wingei the NTS are about double the length of these in the other two species. The transcribed spacers of both K. lactis and H. wingei contain conserved tracts. A homologous sequence of about 60 bp was found in the middle of the external transcribed spacer of H. wingei upon heteroduplexing with S. carlsbergensis rDNA, whereas the sequence at the transcription initiation site itself was insufficiently homologous to form a duplex. The sequence of the homologous region was determined both in H. wingei and K. lactis and compared with that of S. carlsbergensis. The function of this conserved element within the external transcribed spacer is discussed.