Kinase activity associated with caldesmon is Ca2+/calmodulin-dependent kinase II

The relationship of the kinase which co-purifies with caldesmon to Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) was investigated by studying the phosphorylation of bovine brain synapsin I, as well-characterized substrate of CaM-kinase II. Synapsin I is a very good substrate (Km = 90 nM) of the co-purifying kinase, which phosphorylates two sites in synapsin I, both of which are distinct from the single site phosphorylated by cyclic-AMP-dependent protein kinase. Phosphorylation of synapsin I is Ca2(+)- and calmodulin-dependent: half-maximal activation occurs at 0.13 microM-Ca2+ and maximal activity at 0.4 microM-Ca2+. Phosphorylation of the co-purifying kinase slightly enhances the rate, but does not alter the stoichiometry, of subsequent synapsin I phosphorylation; it does, however, circumvent the requirement for Ca2+ and calmodulin. The properties of this kinase therefore closely resemble those of CaM-kinase II, and we conclude that it is probably a smooth-muscle isoenzyme of CaM-kinase II.

Health promotion and self-care in the case of cardiovascular disease

Since the early 1970s experimental studies on self-care and chronic disease have demonstrated the importance of combining professional knowledge and the experiential insight of patients. As a means of promoting health the Duo Formula Group approach (DFG) method has been developed. Part of a larger DFG study focuses on the application and evaluation of a self-care programme following cardiac surgery. The duo formula method teaches patients how to assume more responsibility for the management of their physical condition. This DFG study used an experimental/control group design whereby 59 patients and 49 partners were assessed pre-intervention (M1), post-intervention (M2) and during a follow-up period of 6 months (M3). The study examined changes in illness behaviour, depression, Type A behaviour and anxiety in both patients and partners. At this intermediate stage of the study, patients show decreased depression (M2-M1: t = 2.79, p less than 0.01); and their partners decreased anxiety (M2-M1: t = 1.97, 0.05 less than p less than 0.10).

Monitoring of fracture healing by lateral and axial vibration analysis

The ability to monitor the healing of bone fractures is crucially important in their treatment. The aim of the present study was to develop and validate an objective method for monitoring fracture healing based on bone vibrational response. An analytic model was formulated, with which the mechanical parameters at the fracture site could be studied in relation to both lateral and axial bone vibration. Non-uniformities in the stiffness of the bone at the fracture site can be detected since they produce shifting of the vibration and the phase spectrum and result in strong coupling between the lateral and axial vibration response spectra. The validity of the model was tested in experiments using fresh cadaver tibiae with transverse osteotomy and materials simulating fracture callus. The results of the study of vibration amplitude and phase angle and the coupling of axial and lateral vibration in these experiments confirm our analytic projection. Preliminary results of in vivo investigations using the described method are encouraging.

Carcinoid tumors of the gastrointestinal tract: a 44-year experience

We retrospectively studied 112 patients treated for gastrointestinal carcinoid tumors between 1942 and 1986. The three most common sites were jejunum-ileum (27%), rectum (27%), and appendix (21%), followed by colon (12%), duodenum (9%), and stomach (4%). Seventy-five percent of patients had localized disease, primarily in the rectum and appendix. Colon and small bowel tumors were often manifested with metastatic disease. Complete surgical excision of all localized disease was curative in 94% of patients. Corrected five- and ten-year survival rates were 90% and 58%, respectively, in cases of localized disease, whereas in stage III disease, the rates were 46% and less than 30%, respectively. All patients with liver metastasis died within 25 months. A second malignancy, most often adenocarcinoma of the colon, occurred in 25% of patients. Carcinoid syndrome was found in 6% of patients. The best survival rates were noted for appendiceal and rectal carcinoids, and the worst for colon and stomach carcinoids.

Activation of protein kinase C by the dihydropyridine calcium channel blocker, felodipine

Felodipine, a dihydropyridine Ca2+ channel blocker, appears to have intracellular sites of action in addition to its ability to attenuate voltage-dependent Ca2+ channels in smooth muscle cells. In vitro, felodipine inhibits several calmodulin-dependent enzymes such as myosin light chain kinase, cyclic nucleotide phosphodiesterase and caldesmon kinase [Walsh MP, Sutherland C and Scott-Woo GC, Biochem Pharmacol 37: 1569-1580, 1988]. Such effects may partially explain the relaxant effects of felodipine and related dihydropyridines on vascular smooth muscle. We have examined the effects of felodipine on the activity of another important enzyme which has been implicated in the regulation of the contractile state of smooth muscle, protein kinase C. We chose to use a physiologically relevant substrate of protein kinase C for these studies, viz. platelet P47 protein, rather than the more commonly used lysine-rich histone which is probably not a physiologically important substrate. Protein kinase C and P47 were purified from human platelets and their important structural and functional properties were characterized. Felodipine and the p-chloro analogue of felodipine enhanced both the rate and extent of P47 phosphorylation by protein kinase C. Half-maximal activation was observed at 9.5 microM felodipine and 8.5 microM p-chloro analogue. Activation by felodipine was dependent upon the presence of phospholipid but did not require diacylglycerol. These observations suggest that the pharmacological actions of felodipine and related dihydropyridines may involve activation of protein kinase C in addition to their known effects on voltage-dependent Ca2+ channels and calmodulin-dependent enzymes.

A randomized clinical trial evaluating sequential methotrexate and fluorouracil in the treatment of patients with node-negative breast cancer who have estrogen-receptor-negative tumors

We evaluated the postoperative use of sequential methotrexate and fluorouracil followed by leucovorin in 679 patients with primary breast cancer, histologically negative axillary nodes, and estrogen-receptor-negative (less than 10 fmol) tumors. No survival advantage was observed with this therapy as compared with no postoperative therapy during four years of follow-up (87 percent vs. 86 percent; P = 0.8). However, there was a significant prolongation of disease-free survival among women who received this therapy as compared with those who did not (80 percent vs. 71 percent; P = 0.003). An advantage was observed in both the patients less than or equal to 49 years old and those greater than or equal to 50. At four years, treatment failure was reduced by 24 percent in the younger group and by 50 percent in the older group. The rates of both local and regional and distant metastases were decreased. These benefits, achieved without the use of an alkylating agent, were associated with tolerable side effects. Multivariate analysis testing for potential interactions failed to identify subgroups of patients who did not benefit from the therapy. These results, although promising, do not obviate the need for additional trials to evaluate potentially better regimens of therapy, but they do suggest that sequential methotrexate-fluorouracil should be used in the control arm in such studies. Their use is also justified for the treatment of patients who refuse to participate in clinical trials, provided the patients meet the eligibility criteria of the present study. Since women with tumors too small for conventional analysis of estrogen-receptor and progesterone-receptor concentrations were not included in this study, we do not recommend systemic treatment for them outside of a clinical trial.

Phosphorylation of caldesmon prevents its interaction with smooth muscle myosin

Caldesmon is known to bind to smooth muscle myosin. Ca2+/calmodulin-dependent phosphorylation of caldesmon completely blocks its interaction with myosin. Cleavage of caldesmon at its 2 cysteine residues by 2-nitro-5-thiocyanobenzoic acid (NTCB) occurs initially at one site to yield 108-kDa and 21.2-kDa peptides and subsequently at the second site within the 108-kDa peptide to yield 85-kDa and 23.5-kDa fragments. The 23.5-kDa peptide retains the ability to bind to myosin. The N-terminal (95 kDa) and C-terminal (42 kDa) chymotryptic peptides of caldesmon were isolated and digested with NTCB: the C-terminal actin- and calmodulin-binding peptide was not cleaved, indicating that it does not contain either of the cysteine residues, whereas the 95-kDa N-terminal peptide was cleaved at two sites to yield 56-kDa, 23.5-kDa, and 21.2-kDa fragments. The arrangement of NTCB fragments in caldesmon is, therefore: 21.2 kDa/23.5 kDa/85 kDa from N to C terminus. Digestion of phosphorylated caldesmon with NTCB suggested a single phosphorylation site in the 21.2-kDa peptide and three sites in the 23.5-kDa peptide. These results lead to the development of a model whereby caldesmon may cross-link actin to myosin and such cross-linking is blocked by phosphorylation of caldesmon. This mechanism may explain the formation of reversible "latch bridges" which permit force maintenance at low levels of myosin phosphorylation in intact smooth muscles.

Effects of felodipine (a dihydropyridine calcium channel blocker) and analogues on calmodulin-dependent enzymes

We have examined the effects on the activities of three calmodulin-dependent enzymes (cAMP phosphodiesterase, caldesmon kinase and myosin light chain kinase) of the dihydropyridine Ca2+ channel blocker felodipine and three analogues (p-chloro, oxidized and t-butyl) exhibiting different pharmacological potencies. The cAMP phosphodiesterase was inhibited completely by felodipine and the p-chloro analogue with IC50 values of 3.7 and 1.5 microM respectively. The oxidized and t-butyl analogues were relatively ineffective in inhibiting cAMP phosphodiesterase. Felodipine and the p-chloro analogue inhibited the basal (Ca2+/calmodulin-independent) activity of cAMP phosphodiesterase as well as the calmodulin-stimulated activity. Calmodulin was relatively ineffective in preventing inhibition of cAMP phosphodiesterase by felodipine and the p-chloro analogue. These observations suggest that felodipine may act directly on the phosphodiesterase as well as through calmodulin. Felodipine and the p-chloro analogue inhibited Ca2+/calmodulin-dependent caldesmon kinase with similar potencies (IC50 = 17.4 microM), whereas the oxidized and t-butyl analogues caused no inhibition. Similarly, felodipine and the p-chloro analogue inhibited myosin light chain kinase activity whether the isolated 20 kD light chain (IC50 = 12.6 microM) or intact myosin (IC50 = 11.0 microM) was used as substrate. Inhibition in each case was prevented by excess calmodulin. The oxidized and t-butyl derivatives caused little or no inhibition. Finally, the effects of felodipine and the three analogues on two processes which are dependent on myosin phosphorylation were examined, namely the actin-activated Mg2+-ATPase activity of myosin and the assembly of myosin filaments. Felodipine and the p-chloro analogue inhibited the actin-activated Mg2+-ATPase activity of smooth muscle myosin (IC50 = 25.1 microM). The oxidized and t-butyl analogues exhibited no inhibition. Similarly, felodipine and the p-chloro analogue blocked myosin filament assembly induced by low concentrations of calmodulin, whereas the oxidized and t-butyl analogues did not. Again, inhibition of the actin-activated myosin Mg2+-ATPase and myosin filament assembly by felodipine and the p-chloro analogue could be reversed by raising the calmodulin concentration. These observations suggest that some of the pharmacological actions of felodipine on smooth muscle may involve inhibition of calmodulin-dependent enzymes which are functionally involved in the regulation of smooth muscle contraction.

Holistic physicians and family practitioners: similarities, differences and implications for health policy

Although loosely defined, holistic or alternative medicine has been viewed by most observers as fundamentally at odds with mainstream biomedical approaches. Convergence or integration of the two are seen as highly unlikely. We attempt to assess the potential for such integration empirically through a survey of physicians, members of the American Holistic Medical Association (N = 340) and a comparison group of family practitioners (N = 142). Although social origins of the two groups are similar, they differ in their completion of residency training and a variety of practice characteristics. While the groups differ in the predicted directions in their evaluation and utilization of holistic techniques and in their attitudes toward the nature of medical practice, there is a good deal of overlap. Personal experiences, especially those in the area of religion/spirituality and psychotherapy differ sharply between the groups. Policy concerned with fostering cooperation or convergence between holistic and mainstream medicine should differentiate between clinical attitudes and behaviors (which appear to be more compatible than has been suggested), and the personal world views of physicians (which appear to be much further apart).

Activation of smooth muscle myosin Mg2+-ATPase by native thin filaments and actin/tropomyosin

Application of the myosin competition test (Lehman, W., and Szent-Györgyi, A. G. (1975) J. Gen. Physiol. 66, 1-30) to chicken gizzard actomyosin indicated that this smooth muscle contains a thin filament-linked regulatory mechanism. Chicken gizzard thin filaments, isolated as described previously (Marston, S. B., and Lehman, W. (1985) Biochem. J. 231, 517-522), consisted almost exclusively of actin, tropomyosin, caldesmon, and an unidentified 32-kilodalton polypeptide in molar ratios of 1:1/6:1/26:1/17, respectively. When reconstituted with phosphorylated gizzard myosin, these thin filaments conferred Ca2+ sensitivity (67.8 +/- 2.1%; n = 5) on the myosin Mg2+-ATPase. On the other hand, no Ca2+ sensitivity of the myosin Mg2+-ATPase was observed when purified gizzard actin or actin plus tropomyosin was reconstituted with phosphorylated gizzard myosin. Native thin filaments were rendered essentially free of caldesmon and the 32-kilodalton polypeptide by extraction with 25 mM MgCl2. When reconstituted with phosphorylated gizzard myosin, caldesmon-free thin filaments and native thin filaments exhibited approximately the same Ca2+ sensitivity (45.1 and 42.7%, respectively). The observed Ca2+ sensitivity appears, therefore, not to be due to caldesmon. Only trace amounts of two Ca2+-binding proteins could be detected in native thin filaments. These were identified as calmodulin (present at a molar ratio to actin of 1:733) and the 20-kilodalton light chain of myosin (present at a molar ratio to actin of 1:270). The Ca2+ sensitivity observed in an in vitro system reconstituted from gizzard thin filaments and either skeletal myosin or phosphorylated gizzard myosin is due, therefore, to calmodulin and/or an unidentified minor protein component of the thin filaments which may be an actin-binding protein involved in regulating actin filament structure in a Ca2+-dependent manner.

Organizing Sports Medicine Coverage at the Canada Summer Games

In brief: This article documents the experience with medical coverage for the 1985 Canada Summer Games to help determine the demands of caring for athletes at this level of competition. A total of 599 medical and physiotherapy treatments were given to the 339 members of the British Columbian team (including 20 mission staff) during the two weeks of the games. Most treatments were carried out in the clinic by physiotherapists, suggesting that athletes were seen more frequently for chronic musculoskeletal complaints than for acute medical emergencies. However, physicians responsible for on-site care must be equipped to treat conditions ranging from corneal abrasions to acute head or neck injuries.

Harvey-ras oncogene restriction fragment alleles in familial melanoma kindreds

Unique and uncommon BamHI allelic restriction fragments of the Ha-ras locus have been reported in the genomes of patients with cancer and of three affected members of a familial melanoma kindred (Krontiris et al., 1986). Analysis of the BamHI and Msp/HpaII restriction fragments of peripheral blood leucocyte DNA from the members of two families with hereditary melanoma (HM)/familial dysplastic naevus syndrome (DNS) revealed that the only Ha-ras allele common to four affected members of one kindred and two from a second kindred, was the 6.7kb allele which is found in 66% of the normal population. This allele was found equally in affected and non-affected family members, and in one affected case was inherited from an unaffected homozygous parent. It was absent in two affected sisters in a third kindred. In the first kindred the karyotype of all three melanoma sufferers was 46XX 9qh+, while six unaffected members had a normal karyotype. BamHI polymorphism of the Ha-ras gene does not identify the affected members in the HM/DNS families studied.

Motor activity changes and conditioned taste aversions induced by administration of scopolamine in rats: role of the area postrema

Three experiments examined the effects of centrally and peripherally acting scopolamine (scopolamine hydrochloride-SHC) or only peripherally acting scopolamine (scopolamine methyl nitrate-SMN), on motor activity levels and the ability of these agents to induce taste aversions. In Experiment 1 rats were injected with isotonic saline, 1 mg/kg SHC, or 1 mg/kg SMN. SHC produced significant increases in stabilimeter activity (p less than 0.025) and in rearing response frequency (p less than 0.01), whereas SMN resulted in significantly less activity (p less than 0.025). Both agents induced strong conditioned taste aversions to saccharin (p less than 0.01). Experiment 2 examined the role of the area postrema in mediating these drug induced behavioral changes. Sham lesioned and area postrema lesioned rats were given saline, SHC or SMN (1 mg/kg for both drugs) and examined for changes in activity, rearing response frequency and induction of taste aversions to saccharin. SHC again produced significant increases in activity (p less than 0.01), whereas SMN produced decrements in activity (p less than 0.05). However, the brain lesion did not consistently alter the effects of these drugs on activity but it did reduce the amount of the decrement observed in rearing responses in SMN treated rats. The brain lesion also altered the ability of the drugs to induce taste aversions. Both SMN and SHC produced strong taste aversions in the sham lesioned rats (p less than 0.01) but no significant aversions were observed in the area postrema lesioned rats. Experiment 3 examined the ability of the brain lesion to alter the effects of SMN by using a within groups design. Area postrema lesions were found to attenuate but not abolish, the inhibitory effects of SMN on both activity levels and rearing responses (p less than 0.03). The results of these experiments suggest that in the absence of the chemically-sensitive area postrema both SMN and SHC fail to induce taste aversions and the inhibitory effect of SMN on spontaneous activity is attenuated.

Vascular smooth muscle caldesmon

Caldesmon, a major actin- and calmodulin-binding protein, has been identified in diverse bovine tissues, including smooth and striated muscles and various nonmuscle tissues, by denaturing polyacrylamide gel electrophoresis of tissue homogenates and immunoblotting using rabbit anti-chicken gizzard caldesmon. Caldesmon was purified from vascular smooth muscle (bovine aorta) by heat treatment of a tissue homogenate, ion-exchange chromatography, and affinity chromatography on a column of immobilized calmodulin. The isolated protein shared many properties in common with chicken gizzard caldesmon: immunological cross-reactivity, Ca2+-dependent interaction with calmodulin, Ca2+-independent interaction with F-actin, competition between actin and calmodulin for caldesmon binding only in the presence of Ca2+, and inhibition of the actin-activated Mg2+-ATPase activity of smooth muscle myosin without affecting the phosphorylation state of myosin. Maximal binding of aorta caldesmon to actin occurred at 1 mol of caldesmon: 9-10 mol of actin, and binding was unaffected by tropomyosin. Half-maximal inhibition of the actin-activated myosin Mg2+-ATPase occurred at approximately 1 mol of caldesmon: 12 mol of actin. This inhibition was also unaffected by tropomyosin. Caldesmon had no effect on the Mg2+-ATPase activity of smooth muscle myosin in the absence of actin. Bovine aorta and chicken gizzard caldesmons differed in several respects: Mr (149,000 for bovine aorta caldesmon and 141,000 for chicken gizzard caldesmon), extinction coefficient (E1%280nm = 19.5 and 5.0 for bovine aorta and chicken gizzard caldesmon, respectively), amino acid composition, and one-dimensional peptide maps obtained by limited chymotryptic and Staphylococcus aureus V8 protease digestion. In a competitive enzyme-linked immunosorbent assay, using anti-chicken gizzard caldesmon, a 174-fold molar excess of bovine aorta caldesmon relative to chicken gizzard caldesmon was required for half-maximal inhibition. These studies establish the widespread tissue and species distribution of caldesmon and indicate that vascular smooth muscle caldesmon exhibits physicochemical differences yet structural and functional similarities to caldesmon isolated from chicken gizzard.

Phosphorylation of bovine cardiac C-protein by protein kinase C

C-protein, a thick filament-associated protein, has been isolated from bovine myocardium and found to be a substrate in vitro of the Ca2+- and phospholipid-dependent protein kinase (protein kinase C). Incorporation of approximately 1.6 mol Pi/mol C-protein was observed. This phosphorylation was dependent on both Ca2+ and a phospholipid (L-alpha-phosphatidyl-L-serine was used). Phosphate incorporation specifically into C-protein was verified by SDS-polyacrylamide gel electrophoresis and autoradiography and was almost exclusively into serine residues (86.9%), with only a small amount of phosphothreonine (13.1%) and no phosphotyrosine being detected. Two-dimensional thin-layer electrophoresis of a chymotryptic digest of phosphorylated C-protein indicated site specificity of phosphorylation. Cardiac C-protein is known to be a substrate of cAMP-dependent protein kinase both in vitro and in vivo (Jeacocke, S.A. and England, P.J. (1980) FEBS Lett. 122, 129-132). Isolated bovine cardiac C-protein was rapidly phosphorylated, to the extent of 5 mol/mol, by the purified catalytic subunit of cAMP-dependent protein kinase. Phosphorylation catalyzed by these two protein kinases was not additive, suggesting that the sites phosphorylated by protein kinase C are also phosphorylated by cAMP-dependent protein kinase. Chicken cardiac muscle has also been shown to contain a Ca2+, calmodulin-dependent protein kinase which phosphorylates C-protein (Hartzell, H.C. and Glass, D.B. (1984) J. Biol. Chem. 259, 15587-15596). The physiological role of cardiac C-protein may therefore be subject to regulation by multiple protein kinases.

Conditioned taste aversions induced by 1-5-hydroxytryptophan are mediated by the area postrema

Previous studies have shown that injections of 5-HTP can induce conditioned taste aversions when paired with a novel taste. Adult male albino rats received either lesions of the area postrema or were subjected to a sham lesion procedure. All rats were adjusted to a 23 1/2 hr/day water deprivation schedule and on the conditioning day were given a 0.15% saccharin solution for 1/2 hr. After drinking the saccharin fluid 9 area postrema lesioned and 10 sham lesioned rats were injected i.p. with 25 mg/kg 1-5-hydroxytryptophan. Similarly 10 area postrema lesioned and 6 sham lesioned rats were injected with the vehicle solution. A two-bottle choice test between the saccharin solution and water was given to all animals on the third and fourth days after the conditioning day. The sham lesioned rats injected with the 1-5-hydroxytryptophan exhibited a strong aversion to the saccharin taste whereas the vehicle injected sham lesioned rats showed an equal preference for the two fluids. The difference in group mean saccharin preference ratio was significant (p less than .01). Both area postrema lesioned groups exhibited saccharin preference ratios that were comparable to and not significantly different from the sham lesioned animals injected with the vehicle solution. These results show that an intact area postrema is necessary for induction of conditioned taste aversions with 1-5-hydroxytryptophan.

Dying of cancer. Factors influencing the place of death of patients

An analysis of factors influencing the place of death of patients seen by two medical oncology units is reported. There were 1295 recorded deaths of patients from the Royal Prince Alfred Hospital, Sydney, and 688 from The Royal North Shore Hospital of Sydney during 1979-1981; the places of death were known in 1724 instances (87%). Of these, 73% of patients died in hospital, 9% died in terminal nursing care (TNC) institutions, and the remainder at home. Factors influencing the place of death were the place of residence, age, home circumstances-social support, diagnosis, and interval from first contact with the medical oncology unit. Our data indicate that a complex interrelation of these factors determines the place of death. Improvements in community services may allow more patients with cancer to die at home or in TNC institutions, but a substantial proportion of these will still die in major hospitals. Therefore, there is a need for the improvement of palliative care services in these institutions.

ACTH1-39 but not naltrexone produces biphasic effects on locomotor activity

Two experiments were conducted in order to investigate the effects of chronic ACTH and naltrexone treatment on motor activity in an open-field. In the first experiment, Wistar rats received two daily injections of either ACTH1-39-saline, naltrexone-saline, ACTH1-39-naltrexone or saline-saline for 24 consecutive days. Immediately following injections, motor activity was measured every fourth day. The results indicated that ACTH and naltrexone each had depressive effects on motor activity that did not dissipate over 24 days. In the second experiment, the procedure was similar to the first except that motor activity was measured at five hours postinjection. The results revealed that naltrexone by itself or in combination with ACTH had no observable effect on motor activity. ACTH was observed to have a stimulatory effect on motor activity that decreased over days and was not naltrexone reversible. The results are discussed in terms of different mechanisms underlying the effects of ACTH and naltrexone.

Coexisting lentigo of the larynx and melanoma of the oral cavity: report of a case

A case is presented of coexisting malignant melanoma of the oral cavity and large benign lentigo of the pyriform sinus of the larynx. The absence of a pigmented lesion of the oral cavity in this patient suggests the possibility that lentiginous lesions of the larynx and oral cavity represent simultaneous invasion of these sites by melanocytes later undergoing malignant degeneration in the oral cavity. The histologic observations of these separate lesions are presented and compared. The literature is reviewed briefly. The widespread clinical opinion that malignant melanoma is a systemic disease is illustrated by this case.

Photoreactivating enzyme from Escherichia coli: appearance of new absorption on binding to ultraviolet irradiated DNA

The photoreactivating enzyme, PRE, monomerizes pyrimidine dimers in DNA in a light requiring reaction (lambda greater than 300 nm). However, the purified PRE from E. coli has no well-defined absorption band for lambda greater than 300 nm. Using absorption difference spectroscopy, we show that when PRE is mixed with ultraviolet-irradiated DNA, new absorption appears in the spectral region required for catalysis. There is a concomitant decrease in the absorption of the mixture for wavelength less than 300 nm. The hyperchromicity for lambda greater than 300 nm is true absorption, not an artifact due to light scattering. Both the hyperchromicity (lambda greater than 300 nm) and hypochromicity (lambda less than 300 nm) can be reversed by irradiation of 365 nm with identical first-order kinetics. We estimate the molar extinction coefficient of the new absorption to be 6900 +/- 1400 at 350 nm. We conclude that the PRE from E. coli does not possess a distinct "chromophore" which by itself is entirely responsible for the absorption of photoreactivating light. Instead, new absorption results when PRE binds its substrate, dimer-containing DNA.