Gene duplication and the evolution of silenced chromatin in yeasts.

다국어 초록 (Multilingual Abstract)

In Saccharomyces cerevisiae, proper maintenance of haploid cell identity requires the SIR complex to mediate the silenced chromatin found at the cryptic mating-type loci, HML and HMR. This complex consists of Sir2, a histone deacetylase, and the histone binding proteins Sir3 and Sir4. Interestingly, both Sir2 and Sir3 have paralogs from a genome duplication that occurred after the divergence of Saccharomyces and Kluyveromyces species, approximately 100 million years ago. The histone deacetylase HST1 is the paralog of SIR2 and works with the promoter-specific SUM1 complex to repress sporulation and alpha-specific genes. ORC1 is the paralog of SIR3 and is an essential subunit of the Origin Recognition Complex and also recruits SIR proteins to the HM loci. I have investigated the functions of these proteins in the non-duplicated species Kluyveromyces lactis and compared these functions to those found in S. cerevisiae.
I have shown that SIR2 and HST1 subfunctionalized post-duplication via the duplication, degeneration and complementation mechanism. In S. cerevisiae, Sir2 and Hst1 have non-overlapping functions in a wild-type strain background, yet Sir2 has retained the ability to function like Hst1 and contribute to the repression of sporulation genes in an hst1Delta strain. I have also shown, with a chimeric Sir2-Hst1 protein, that distinct specificity domains have diverged between Sir2 and Hst1 that specify a Sir2 interaction with the SIR complex and an Hst1 interaction with the SUM1 complex. Trans-species complementation assays show that the non-duplicated Sir2 from K. lactis can interact with both SIR and SUM1 complexes in S. cerevisiae.
Further analysis into the non-duplicated experimental system of K. lactis has revealed that KlSir2 functions like both ScSir2 and ScHst1 as deletion of KlSIR2 de-represses the HM loci as well as sporulation and cell-type specific genes. A physical interaction between KlSir2 and the histone binding protein KlSir4 is conserved in K. lactis , and both proteins spread across the HML locus and associate with telomeres in a manner similar to S. cerevisiae. KlSir2 also physically interacts with the DNA-binding protein, KlSum1, to repress sporulation and cell-type specific genes in a promoter-specific manner and recruitment of KlSir2 to these loci is dependent on KlSum1. Surprisingly, deletion of KlSUM1 also de-represses HML and HMR, a phenotype not observed in S. cerevisiae. I show by chromatin immunoprecipitation that KlSum1 directly regulates the HM loci in K. lactis by spreading across these regions in a mechanism that is distinct from its role in repressing sporulation-specific genes. This result indicates that KlSum1 is a key regulator of not only meiotic, but also mating-type transcriptional programming.
The SIR3-ORC1 gene pair has previously been used as an example of neofunctionalization based on accelerated rates of evolution. However, my studies of KlOrc1 show it is distributed across HML and associates with Sir2 and Sir4 at telomeres, indicative of it having Sir3-like capabilities to spread across chromatin. This ability of KlOrc1 to spread is distinct from its functions with ORC, and is entirely dependent on it's BAH domain, a domain which is capable of binding histones in S. cerevisiae both in vivo and in vitro. These findings demonstrate that prior to the genome duplication there was a silencing complex that contained both KlSir2 and KlOrc1. In addition to their functions at HML and the telomeres, KlOrc1 associates with replication origins and KlSir2 and KlSum1 work in complex to repress sporulation genes in a promoter-specific manner. The multiple functions of both KlOrc1 and KlSir2 in K. lactis indicate that after duplication, these properties were divided among paralogs and subsequently specialized to perform the functions that have been characterized in S. cerevisiae.