A physical map of the polytenized region (101EF-102F) of chromosome 4 in Drosophila melanogaster

Chromosome 4, the smallest autosome (similar to 5 Mb in length) in Drosophi la melanogaster contains two major regions. The centromeric domain (similar to 4 Mb) is heterochromatic and consists primarily of short, satellite rep eats. The remaining similar to 1.2 Mb, which constitutes the banded region (101E-102F) on salivary gland polytene chromosomes and contains the identif ied genes, is the region mapped in this study. Chromosome walking was :hind ered by the abundance of moderately repeated sequences dispersed along the chromosome, so we used many entry points to recover overlapping cosmid and BAC clones. rn situ hybridization of probes from the two ends of the map to polytene chromosomes confirmed that the cloned region had spanned the 101E -102F interval. Our BAC clones comprised three contigs; one gap was positio ned distally in 102EF and the other was located proximally at 102B. Twenty- three genes, representing about half of our revised estimate of the total n umber of genes on chromosome 4, were positioned on the BAC contigs. A minim al ti:ling set of the clones we have mapped will facilitate both the assemb ly of the DNA sequence of die chromosome and a functional analysis of its g enes.