Identification and characterization of histone gene amplification as a method for dosage compensation in Saccharomyces cerevisiae

Gene amplification is a process that increases the copy number of a gene or genomic region to two or more. Many organisms utilize gene amplification in response to particular developmental stages, environmental stresses, or decreased levels of a gene product. This dissertation describes the identification and characterization of a previously unknown mechanism to amplify histone gene copy number in response to reduced histone levels in Saccharomyces cerevisiae . Specifically, the experiments focus on the two gene pairs that encode histones H2A and H2B, HTA1-HTB1 and HTA2-HTB2 . Our findings demonstrate HTA2-HTB2 amplifies to dosage compensate for reduced histone H2A-H2B levels after a deletion of HTA1-HTB1 . Formed from an enhanced recombination between two Ty1 retrotransposable elements that flank the HTA2-HTB2 locus on chromosome II, this stable circular amplification contains origins of replication, a centromere, and the histone H3-H4 locus HHT1-HHF1 . In addition to forming at a frequency higher than is observed for typical gene duplications, the HTA2-HTB2 amplification is required for ( hta1-htb1 )Δ viability. Our further analysis of the amplification event demonstrates that upon introduction of replication fork pauses, wild-type cells can stimulate formation of the HTA2-HTB2 amplification without affecting recombination events between other Ty1 elements. This induction of the amplification event requires the presence of replication origins within the amplified region. In addition, our data suggests that altered histone stoichiometry can induce the HTA2-HTB2 amplification event. Taken together, this dissertation indicates that cells can utilize replication fork pauses to specifically enhance a Ty1-Ty1 recombination event, such as the one that forms the histone gene amplification, as an adaptive response to reduced histone levels or other environmental signals.