Sets of diagnostic Southern blots are presented in Fig. 2
and demonstrate that the in vitro deletions were correctly
shuttled into the corresponding histone loci. In the copy-I
results of Fig. 2 A, for example, the Hind III digestions of
whole genome DNA from wild-type and copy-II deletion
strains show the normal 6.8-kb copy-I restricton fragment
when probed with Sc191. However, in the copy-I deletion
strain this fragment is ,~2 kb smaller. The other restriction
enzyme digestions in Fig. 2 A show that the deletion spans
the H3- and H4-transcribed sequences and that a new Eco
RI linker has been inserted at the site of the deletion. Since
the Sc191 fragment used as the probe in Fig. 2 A contains the
H3 and H4 coding sequences, it also crosshybridizes with
the coding sequences of the copy-II locus. These copy-II
fragments are present in the DNA digests from wild-type and
copy-I deletion strains, but are absent in the copy-II deletion
strain indicating that the coding sequences are missing from
the putative copy-II deletion. Positive evidence for the deletion
is presented in Fig. 2 B. The hybridization probe in this
case was the Sc218 fragment that is specific for the copy-II
downstream sequences. In the DNA from the copy-II deletion
strain the normal 3.5-kb Sc2ff/Eco RI fragment is fused
with an H3-distal fragment by the deletion to a give a 9.0-kb
fragment seen in the Eco RI digestions, while it is truncated
with the new Barn HI linker at the site of the deletion to give
a 3.0-kb fragment seen in the Eco RI-Bam HI double digestions.
Thus, haploids containing deletions of either the copy-I or
the copy-II histone H3-H4 gene pairs were successfully constructed.
If deletion of either the copy-I or copy-II genes had
been lethal, dissection of the pink variant diploids would
have produced two live white spore colonies and two dead
spores from the segregants with the deletion chromosome.
We conclude that the copy-I and copy-II histone H3-H4 loci
are functionally redundant; the presence of either region
alone is sufficient for cell viability. This is in agreement with
the results for disruption of the duplicated H2A-H2B loci
(27, 39).