The replicative polymerase δ (Polδ), consisting of four subunits, plays a pivotal role in chromosomal replication. Pold4, the smallest subunit of Polδ, is believed to contribute to the regulation of replication by facilitating repair in response to DNA damage. However, that contribution has not been fully elucidated. We here show that Pold4 contributes to the suppression of gene conversion in immunoglobulin-variable (IgV) gene diversification in the chicken DT40 lymphocyte cell line, where gene conversion diversifies the IgV gene through intragenic homologous recombination (HR) between diverged pseudo-V segments. IgV gene conversion is initiated by activation-induced cytidine deaminase-mediated uracil formation in the IgV gene, which in turn converts into an abasic site, leading to replication arrest. POLD4^−/− cells exhibited an increased rate of IgV gene conversion. Moreover, the gene-conversion tract was lengthened and the usage of pseudo-V segments was altered, showing a preference, to use the diverged sequence as a donor in POLD4^−/− cells. These data suggest that Pold4 is involved in the regulation of HR-mediated gene conversion in IgV diversification. By contrast, the rate in HR-mediated, sister-chromatid exchange and gene-targeting induced by an I-SceI endonclease-mediated DNA double-strand break exhibited by POLD4^−/− cells was indistinguishable from that by wild-type cells. These findings indicate that the functionality of general HR is preserved in POLD4^−/− cells. In conclusion, Pold4 is involved in the suppression of IgV-gene conversion without affecting the general functionality of HR.

复制聚合酶 δ (Polδ) 由四个亚基组成，在染色体复制中起着关键作用。Pold4 是 Polδ 的最小亚基，据信通过促进 DNA 损伤的修复来调节复制。然而，这种贡献尚未完全阐明。我们在这里表明 Pold4 有助于抑制鸡 DT40 淋巴细胞细胞系中免疫球蛋白可变 ( IgV ) 基因多样化中的基因转化，其中基因转化通过发散的假 V 片段之间的基因内同源重组 (HR)使IgV基因多样化。IgV结构基因转化是通过活化诱导的胞苷脱氨酶启动-介导的尿嘧啶形成在IgV结构基因，这反过来转化为无碱基位点，导致复制停滞。POLD4 ^-/-细胞表现出增加的IgV基因转化率。此外，基因转换道被加长并且伪V区段的使用被改变，显示出偏好使用分化的序列作为POLD4 ^-/-细胞中的供体。这些数据表明 Pold4 参与了IgV多样化中 HR 介导的基因转换的调节。相比之下，POLD4 ^-/-表现出由I- Sce I 内切酶介导的 DNA 双链断裂诱导的 HR 介导的姐妹染色单体交换和基因靶向的速率^。细胞与野生型细胞无法区分。这些发现表明一般 HR 的功能在POLD4 ^-/-细胞中得以保留。总之，Pold4 参与抑制IgV基因转化而不影响 HR 的一般功能。