A number of yeast S. cerevisiae nuclear gene products
are involved in mitochondrial DNA metabolism, including the DNA helicase
Hmi1p. We have studied the biochemical functions of the Hmi1p using
mtDNA analysis on 2D agarose gels. mtDNA isolated from rho-
hypersuppressive strains is mostly organized as linear concatomers,
circles and different branched species. The most prominent Hmi1p
dependent change is significant increase in rho- mtDNA concatomer length
or repeat number. Also, rho- mtDNA isolated from hmi1 defective strains
seems to contain very little ssDNA, and to have a different pattern of
branched molecules. Analysis of replication intermediates using
restriction enzymes that cut once per mtDNA repeat demonstrates, that
fast leading strand synthesis takes place in strains with functional
Hmi1p. However, the typical replication fork helicase role of the Hmi1p
in yeast mitochondria is unlikely, as the protein is not required for
rho- DNA maintenance, and several Hmi1p mutants that lack ATPase
activity can support wt mtDNA replication. The Hmi1 helicase can form a
strand specific complex with a mt ori sequence element. We propose that
the Hmi1p functions as a stimulatory factor for the mtDNA polymerase
during replication of palindromic sequences.