DNA Processing Proteins Involved in the UV-Induced Stress Response of Sulfolobales

The ups operon of Sulfolobus species is highly induced upon UV stress. Previous studies showed that the pili encoded by this operon are involved in cellular aggregation, which is essential for subsequent DNA exchange between cells, resulting in homologous recombination. The presence of this pilus system increases the fitness of Sulfolobus cells under UV light-induced stress conditions, as the transfer of DNA takes place in order to repair UV-induced DNA lesions via homologous recombination. Four conserved genes (saci_1497 to saci_1500) which encode proteins with putative DNA processing functions are present downstream of the ups operon. In this study, we show that after UV treatment the cellular aggregation of strains with saci_1497, saci_1498, and saci_1500 deletions is similar to that of wild-type strains; their survival rates, however, were reduced and similar to or lower than those of the pilus deletion strains, which could not aggregate anymore. DNA recombination assays indicated that saci_1498, encoding a ParB-like protein, plays an important role in DNA transfer. Moreover, biochemical analysis showed that the endonuclease III encoded by saci_1497 nicks UV-damaged DNA. In addition, RecQ-like helicase Saci_1500 is able to unwind homologous recombination intermediates, such as Holliday junctions. Interestingly, a saci_1500 deletion mutant was more sensitive to UV light but not to the replication-stalling agents hydroxyurea and methyl methanesulfonate, suggesting that Saci_1500 functions specifically in the UV damage pathway. Together these results suggest a role of Saci_1497 to Saci_1500 in the repair or transfer of DNA that takes place after UV-induced damage to the genomic DNA of Sulfolobus acidocaldarius.

Sulfolobales species increase their fitness after UV stress by a UV-inducible pilus system that enables high rates of DNA exchange between cells. Downstream of the pilus operon, three genes that seem to play a role in the repair or transfer of the DNA between Sulfolobus cells were identified, and their possible functions are discussed. Next to the previously described role of UV-inducible pili in the exchange of DNA, we have thereby increased our knowledge of DNA transfer at the level of DNA processing. This paper therefore contributes to the overall understanding of the DNA exchange mechanism among Sulfolobales cells.