MAPPING CHROMOSOME LANDMARKS IN THE CENTROMERE-I REGION OF NEUROSPORA-CRASSA

Chromosome translocation breakpoints, RFLP heterozygosity in partial c hromosome duplications, and RFLP-marked crossover events have been use d as chromosomal landmarks to find the position and orientation of clo ned regions flanking centromere I of Neurospora crassa. Determination of physical:genetic ratios in genomic regions flanking the loci mei-3, un-2, and his-2 supports previous evidence indicating that recombinat ional activity is lower in regions flanking centromere I than in the g eneral N. crassa genome. The homogeneous distribution of crossover eve nts found in these regions suggests that there is not a gradient of cr ossover inhibition in the vicinity of centromere I. Thus, a largely ex tended centromeric effect and/or a general crossover inhibitory effect operating on linkage group I (LGI) could constitute the basis of thes e abnormal physical:genetic ratios. A DNA element containing about 76% A + T was isolated from the centromeric end of a cloned region on LGI R. The fragment includes a previously undescribed DNA sequence, highly repeated in the Neurospora genome, which may correspond to centromeri c DNA. (C) 1997 Academic Press.