The occurrence of DNA double strand breaks induces cell cycle arrest in mortal and immortal human cells. In normal, mortal ®broblasts this block to proliferation is permanent. It depends on the growth regulator p53 and a protein p53 induces, the cyclin dependent kinase inhibitor, p21. We show here that following DNA damage in mortal ®broblasts, the induction of p21 and p53 is to a large degree shortlived. By 8 days after a brief exposure to DNA strand breaking agents, bleomycin or actinomycin D, p53 protein is at baseline levels, while the p53 transactivation level is only slightly above its baseline. By this time the concentration of p21 protein, which goes up as high as 100-fold shortly after treatment, is down to just 2 ± 4-fold over baseline levels. Following the drop in p21 concentration a large increase in the expression level of the tumor suppressor gene p16INK4a is observed. This scenario, where a transient increase in p21 is followed by a delayed induction of p16 INK4a , also happens with the permanent arrest that occurs with cellular senescence. In fact, these cells treated with agents that cause DNA double strand breaks share a number of additional markers with senescent cells. Our ®ndings indicate that these cells are very similar to senescent cells and that they have additional factor(s) beside p21 and p53 that maintain cell cycle arrest.

It is shown that vibrational coherence modulates the femtosecond kinetics of stimulated emission and absorption of reaction centers of purple bacteria. In the DLL mutant of Rhodobacter capsUlatus, which lacks the bacteriopheophytin electron acceptor, oscillations with periods of -500 fs and possibly also of :2 ps were observed, which are associated with formation of the excited state. The kinetics, which reflect primary processes in Rhodobacter sphaeroides R-26, were modulated by oscillations with a period of 700 fs at 796 nm and =z2 ps at 930 nm. In the latter case, at 930 nm, where the stimulated emission of the excited state, P*, is probed, oscillations could only be resolved when a sufficiently narrow (10 nm) and concomitantly long pump pulse was used. This may indicate that the potential energy surface of the excited state is anharmonic or that low-frequency oscillations are masked when higher frequency modes are also coherently excited, or both. The possibility is discussed that the primary charge separation may be a coherent and adiabatic process coupled to low-frequency vibrational modes.The quantum efficiency of the initial charge-separation process in the reaction center of photosynthetic bacteria is near unity because of the combination of the extreme rapidity of forward electron transfer and the low probability of back reaction. In conventional electron-transfer theory (1, 2), it is assumed that vibrational relaxation takes place on a time scale faster than electron transfer and that electron transfer is essentially nonadiabatic. It may be questioned whether these assumptions are justified for the ultrafast initial reactions taking place in the bacterial reaction center. In particular, electron transfer from an excited state that is not completely vibrationally relaxed (3) may be at the origin of the high quantum yield of charge separation. In this case, vibrational coherence of modes coupled to electron transfer is not necessarily lost on the time scale of the reaction. Recently, theoretical studies have appeared in which it has indeed been suggested that vibrational coherence or coherence in the electronic coupling, or both, play a role in primary electron transfer (4-7).If the vibrational relaxation takes place on a time scale that is not significantly faster than electron transfer, lowfrequency (<100 cm-') vibrations may interfere with the charge-separation reaction. When such vibrations are coherent, oscillations may in principle be observed in the optical transients of the involved electronic state(s). Here we report the observation of coherent processes associated with the excited state in the photosynthetic reaction center.The reaction center of purple bacteria, which are used in this study, normally contains four bacteriochlorophylls [two of which display strong dimeric interaction and are designated P (for "special pair") and two of which, BL and BM, are more monomer-like], two bacteriopheophytins (HL and HM), and two quinones (QA and QB) bound to the protein subunits L and M (8, 9)...