The other strategy is to inhibit or eliminate the NHEJ pathway, thereby forcing the transformed DNA to be integrated via HR. With this approach, the frequency of HR has been found to be significantly improved with many reports of success in recent years through the disruption of NHEJ pathway by deleting one or more of its key components [12]. In eukaryotes, the main component of the NHEJ system is the DNA-dependent protein kinase (DNA-PK), a three-protein complex consisting of the DNA-dependent

protein kinase catalytic subunit (DNA-PKcs) and the regulatory DNA-binding subunits, the Ku70/80 heterodimer [14]. The Ku heterodimer is an abundant nonspecific DNA-binding protein Selleckchem Panobinostat comprising of two tightly-associated subunits of about 70 and 83 kDa, named Ku70 and Ku80 GW4869 in vivo respectively [15]. Both proteins exist in organisms ranging from fungi to human, and are arguably the defining proteins of NHEJ because of their sequence conservation [16]. Here, we report the isolation and characterization of KU70 and KU80 homologs in R. toruloides and the evaluation of a KU70-deficient mutant strain generated for improving

gene deletion efficiency in R. toruloides. Results Isolation and characterization of Ku70 and Ku80 encoding genes in R. toruloides Putative genes encoding the Ku70 and Ku80 homologues in the Rhodotorula glutinis ATCC 204091 (now re-named as Rhodosporidium toruloides ATCC 204091) genome were identified by tBLASTn search against the R. glutinis AMN-107 nmr ATCC 204091 genome database at NCBI using the Ustilago maydis Ku70 and Ku80 sequences as the query (GenBank acc. no. XP_761295 and XP_761903 respectively). 5′ and 3′ RACEs were performed to obtain the full-length cDNA sequences. The KU70 cDNA contains a 2,118-nt open reading frame (ORF) flanked by 57-nt and 99-nt 5′ and 3′ untranslated region (UTR) respectively, while the KU80 cDNA contains a 2,766-nt ORF with 76-nt 5′ UTR and 83-nt 3′ UTR. Comparison of the cDNAs with the genomic sequences revealed that the KU70 mRNA spans over 3,047 bp containing 16 exons separated by 15 introns, whereas the KU80 mRNA spans over 3,426 bp Glycogen branching enzyme containing 11 exons separated by 10 introns (Figure 1). All intronic sequences conformed

strictly to the GT-AG rule [17], with a GC content of approximately 61%, which is not significantly different to that of exonic sequences (Table 1). Sequencing of the 3,047 bp KU70 genomic region in R. toruloides ATCC 10657 revealed 100% identity to that of R. toruloides ATCC 204091. A comparison with a number of other fungal homologues are shown in Table 1, which shows that R. toruloides KU70 and KU80 genes have the highest GC content and highest density of introns (1 in 196 nt on average). Figure 1 Genomic organization of KU70 / 80 from R. toruloides . (A) Genomic organization of KU70. (B) Genomic organization of KU80. Exons (indicated by black boxes) were identified by comparing the cDNAs and their corresponding genomic DNA sequences.