ORGANIZATION OF CHROMOSOME ENDS IN USTILAGO-MAYDIS - RECQ-LIKE HELICASE MOTIFS AT TELOMERIC REGIONS

In this study we have established the structure of chromosome ends in the basidiomycete fungus Ustilago maydis. We isolated and characterize d several clones containing telomeric regions and found that as in oth er organisms, they consist of middle repeated DNA sequences. Two princ ipal types of sequence were found: UTASa was highly conserved in nucle otide sequence and located almost exclusively at the chromosome ends, and UTASb was less conserved in nucleotide sequence than UTASa and fou nd not just at the ends but highly interspersed throughout the genome. Sequence analysis revealed that UTASa encodes an open reading frame c ontaining helicase motifs with the strongest homology to RecQ helicase s; these are DNA helicases whose function involves the maintenance of genome stability in Saccharomyces cerevisiae and in humans, and the su ppression of illegitimate recombination in Escherichia coli. Both UTAS a and UTASb contain a common region of about 300 bp located immediatel y adjacent to the telomere repeats that are also found interspersed in the genome. The analysis of the chromosome ends of U. maydis provides information on the general structure of chromosome ends in eukaryotes , and the putative RecQ helicase at UTASa may reveal a novel mechanism for the maintenance of chromosome stability.