Poly-ADP-ribosyltransferase that mediates poly-ADP-ribosylation of proteins and plays a key role in DNA repair. Mediates glutamate, aspartate, serine or tyrosine ADP-ribosylation of proteins: the ADP-D-ribosyl group of NAD(+) is transferred to the acceptor carboxyl group of target residues and further ADP-ribosyl groups are transferred to the 2'-position of the terminal adenosine moiety, building up a polymer with an average chain length of 20-30 units. Serine ADP-ribosylation of proteins constitutes the primary form of ADP-ribosylation of proteins in response to DNA damage. Mainly mediates glutamate and aspartate ADP-ribosylation of target proteins in absence of HPF1. Following interaction with HPF1, catalyzes serine ADP-ribosylation of target proteins; HPF1 conferring serine specificity by completing the PARP1 active site. Also catalyzes tyrosine ADP-ribosylation of target proteins following interaction with HPF1. PARP1 initiates the repair of DNA breaks: recognizes and binds DNA breaks within chromatin and recruits HPF1, licensing serine ADP-ribosylation of target proteins, such as histones, thereby promoting decompaction of chromatin and the recruitment of repair factors leading to the reparation of DNA strand breaks. In addition to base excision repair (BER) pathway, also involved in double-strand breaks (DSBs) repair: together with TIMELESS, accumulates at DNA damage sites and promotes homologous recombination repair by mediating poly-ADP-ribosylation. Mediates the poly(ADP-ribosyl)ation of a number of proteins, including itself, APLF and CHFR. In addition to proteins, also able to ADP-ribosylate DNA: catalyzes ADP-ribosylation of DNA strand break termini containing terminal phosphates and a 2'-OH group in single- and double-stranded DNA, respectively. Required for PARP9 and DTX3L recruitment to DNA damage sites. PARP1-dependent PARP9-DTX3L-mediated ubiquitination promotes the rapid and specific recruitment of 53BP1/TP53BP1, UIMC1/RAP80, and BRCA1 to DNA damage sites. Acts as a regulator of transcription: positively regulates the transcription of MTUS1 and negatively regulates the transcription of MTUS2/TIP150. Plays a role in the positive regulation of IFNG transcription in T-helper 1 cells as part of an IFNG promoter-binding complex with TXK and EEF1A1. Involved in the synthesis of ATP in the nucleus, together with NMNAT1, PARG and NUDT5. Nuclear ATP generation is required for extensive chromatin remodeling events that are energy-consuming. NAD(+) + (ADP-D-ribosyl)n-acceptor = nicotinamide + (ADP-D-ribosyl)n+1-acceptor + H(+). L-seryl-[protein] + NAD(+) = H(+) + nicotinamide + O-(ADP-D-ribosyl)-L-seryl-[protein] L-aspartyl-[protein] + NAD(+) = 4-O-(ADP-D-ribosyl)-L-aspartyl-[protein] + nicotinamide L-glutamyl-[protein] + NAD(+) = 5-O-(ADP-D-ribosyl)-L-glutamyl-[protein] + nicotinamide L-tyrosyl-[protein] + NAD(+) = H(+) + nicotinamide + O-(ADP-D-ribosyl)-L-tyrosyl-[protein] Homo- and heterodimer with PARP2. Interacts with APTX (By similarity). Component of a base excision repair (BER) complex, containing at least XRCC1, PARP1, PARP2, POLB and LRIG3 (By similarity). Interacts with SRY (By similarity). The SWAP complex consists of NPM1, NCL, PARP1 and SWAP70. Interacts with TIAM2 (By similarity). Interacts with PARP3; leading to activate PARP1 in absence of DNA (By similarity). Interacts (when poly-ADP-ribosylated) with CHD1L (via macro domain). Interacts with the DNA polymerase alpha catalytic subunit POLA1; this interaction functions as part of the control of replication fork progression. Interacts with EEF1A1 and TXK. Interacts with RNF4. Interacts with RNF146. Interacts with ZNF423. Interacts with APLF. Interacts with SNAI1 (via zinc fingers); the interaction requires SNAI1 to be poly-ADP-ribosylated and non-phosphorylated (active) by GSK3B. Interacts (when poly-ADP-ribosylated) with PARP9 (By similarity). Interacts with NR4A3; activates PARP1 by improving acetylation of PARP1 and suppressing the interaction between PARP1 and SIRT1 (By similarity). Interacts (via catalytic domain) with PUM3; the interaction inhibits the poly-ADP-ribosylation activity of PARP1 and the degradation of PARP1 by CASP3 following genotoxic stress. Interacts (via the PARP catalytic domain) with HPF1. Interacts with ZNF365. Interacts with RRP1B. Interacts with TIMELESS; the interaction is direct. Interacts with CGAS; leading to impede the formation of the PARP1-TIMELESS complex (By similarity). Interacts with KHDC3, the interaction is increased following the formation of DNA double-strand breaks (By similarity). Localizes to sites of DNA damage. The N-terminal disordered region does not act as a key DNA-binding domain. The WGR and PARP catalytic domains function together to recruit PARP1 to sites of DNA breaks. The N-terminal disordered region is only required for activation on specific types of DNA damage. The WGR domain bridges two nucleosomes, with the broken DNA aligned in a position suitable for ligation. The bridging induces structural changes in PARP1 that signal the recognition of a DNA break to the catalytic domain of PARP1, promoting HPF1 recruitment and subsequent activation of PARP1, licensing serine ADP-ribosylation of target proteins. Poly-ADP-ribosylated on glutamate and aspartate residues by autocatalysis. Poly-ADP-ribosylated by PARP2; poly-ADP-ribosylation mediates the recruitment of CHD1L to DNA damage sites. ADP-ribosylated on serine by autocatalysis; serine ADP-ribosylation takes place following interaction with HPF1 (By similarity). Auto poly-ADP-ribosylated on serine residues, leading to dissociation of the PARP1-HPF1 complex from chromatin (By similarity). Mono-ADP-ribosylated at Lys-523 by SIRT6 in response to oxidative stress, promoting recruitment to double-strand breaks (DSBs) sites (By similarity). S-nitrosylated, leading to inhibit transcription regulation activity. Phosphorylated by PRKDC and TXK. Belongs to the ARTD/PARP family.