Halobacterium halobium contains two gas vacuole protein genes that are located 
in plasmid pHH1 (p-vac) and in the chromosomal DNA (c-vac). The mutation 
frequency for these genes is different: the constitutively expressed p-vac gene 
is mutated with a frequency of 10(-2), while the chromosomal gene expressed in 
the stationary phase of growth is mutated with a frequency of 10(-5). The 
difference in the mutation susceptibility is due to the dynamics of plasmid 
pHH1. p-vac gene mutations are caused (i) by the integration of an insertion 
element or (ii) by a deletion event encompassing the p-vac gene region. In 
contrast, c-vac mutants analyzed to date incurred neither insertion elements nor 
deletions. Deletion events within pHH1 occur at high frequencies during the 
development of a H. halobium culture. The investigation of the fusion regions 
resulting from deletion events indicates that insertion elements are involved. 
The analysis of pHH1 deletion variants led to a 4 kilobase pair DNA region 
containing the origin of replication of the pHH1 plasmid.