1. ¹Fundación Instituto Leloir, CONICET, Buenos Aires, Argentina
2. ²Department of Biological Sciences, Columbia University, New York, NY, USA

Correspondence: Dr V Gottifredi, Fundación Instituto Leloir, Buenos Aires 1405, Argentina. E-mail: vgottifredi@leloir.org.ar

Received 19 July 2005; Revised 24 October 2005; Accepted 9 November 2005; Published online 9 January 2006.

Abstract

p21^Cip1/WAF1 is a known inhibitor of the short-gap filling activity of proliferating cell nuclear antigen (PCNA) during DNA repair. In agreement, p21 degradation after UV irradiation promotes PCNA-dependent repair. Recent reports have identified ubiquitination of PCNA as a relevant feature for PCNA-dependent DNA repair. Here, we show that PCNA ubiquitination in human cells is notably augmented after UV irradiation and other genotoxic treatments such as hydroxyurea, aphidicolin and methylmethane sulfonate. Intriguingly, those DNA damaging agents also promoted downregulation of p21. While ubiquitination of PCNA was not affected by deficient nucleotide excision repair (NER) and was observed in both proliferating and arrested cells, stable p21 expression caused a significant reduction in UV-induced ubiquitinated PCNA. Surprisingly, the negative regulation of PCNA ubiquitination by p21 does not depend on the direct interaction with PCNA but requires the cyclin dependent kinase binding domain of p21. Taken together, our data suggest that p21 downregulation plays a role in efficient PCNA ubiquitination after UV irradiation.