The differential hybridization between maize and
sorghum can be used as a tool in the identification of single- and
low-copy sequences. In maize, the small arm of chromosome 4 contains a
large cluster of 20–22 pseudogenes and genes encoding α-zeins, the
major group of storage proteins in kernels. Most of the intergenic
region in the cluster consists of highly repetitive DNA, many of them
retrotransposons (V.L., R. Wing, and J.M., unpublished work). BACs
containing sorghum DNA homologous to the 22-kDa cluster were hybridized
to 14 cosmid clones from the cluster region isolated from a maize
BSSS53 library. (A) Agarose gel electrophoresis of the maize
cosmids digested with EcoRI. (B and
C) Southern blots from the same gel, hybridized to two
different sorghum BACs, M18 and J21, respectively. Fragments
cross-hybridizing to sorghum BACs contained single- or low-copy
sequences, including zeins and the RFLP marker php20725.