The expression of the Escherichia coli DNA polymerases pol V (UmuD'2C complex) and pol IV (DinB) increases in response to DNA damage. The induction of pol V is accompanied by a substantial increase in mutations targeted at DNA template lesions in a process called SOS-induced error-prone repair. Here we show that the common DNA template lesions, TT (6-4) photoproducts, TT cis-syn photodimers and abasic sites, are efficiently bypassed within 30 seconds by pol V in the presence of activated RecA protein (RecA*), single-stranded binding protein (SSB) and pol III's processivity beta,gamma-complex. There is no detectable bypass by either pol IV or pol III on this time scale. A mutagenic 'signature' for pol V is its incorporation of guanine opposite the 3'-thymine of a TT (6-4) photoproduct, in agreement with mutational spectra. In contrast, pol III and pol IV incorporate adenine almost exclusively. When copying undamaged DNA, pol V exhibits low fidelity with error rates of around 10(-3) to 10(-4), with pol IV being 5- to 10-fold more accurate. The effects of RecA protein on pol V, and beta,gamma-complex on pol IV, cause a 15,000- and 3,000-fold increase in DNA synthesis efficiency, respectively. However, both polymerases exhibit low processivity, adding 6 to 8 nucleotides before dissociating. Lesion bypass by pol V does not require beta,gamma-complex in the presence of non-hydrolysable ATPgammaS, indicating that an intact RecA filament may be required for translesion synthesis.

SUMMARY Heme is the prosthetic group for cytochromes, which are directly involved in oxidation/reduction reactions inside and outside the cell. Many cytochromes contain heme with covalent additions at one or both vinyl groups. These include farnesylation at one vinyl in hemes o and a and thioether linkages to each vinyl in cytochrome c (at CXXCH of the protein). Here we review the mechanisms for these covalent attachments, with emphasis on the three unique cytochrome c assembly pathways called systems I, II, and III. All proteins in system I (called Ccm proteins) and system II (Ccs proteins) are integral membrane proteins. Recent biochemical analyses suggest mechanisms for heme channeling to the outside, heme-iron redox control, and attachment to the CXXCH. For system II, the CcsB and CcsA proteins form a cytochrome c synthetase complex which specifically channels heme to an external heme binding domain; in this conserved tryptophan-rich “WWD domain” (in CcsA), the heme is maintained in the reduced state by two external histidines and then ligated to the CXXCH motif. In system I, a two-step process is described. Step 1 is the CcmABCD-mediated synthesis and release of oxidized holoCcmE (heme in the Fe+3 state). We describe how external histidines in CcmC are involved in heme attachment to CcmE, and the chemical mechanism to form oxidized holoCcmE is discussed. Step 2 includes the CcmFH-mediated reduction (to Fe+2) of holoCcmE and ligation of the heme to CXXCH. The evolutionary and ecological advantages for each system are discussed with respect to iron limitation and oxidizing environments.

Error-free lesion bypass and error-prone lesion bypass are important cellular responses to DNA damage during replication, both of which require a DNA polymerase (Pol). To identify lesion bypass DNA polymerases, we have purified human Polkappa encoded by the DINB1 gene and examined its response to damaged DNA templates. Here, we show that human Polkappa is a novel lesion bypass polymerase in vitro. Purified human Polkappa efficiently bypassed a template 8-oxoguanine, incorporating mainly A and less frequently C opposite the lesion. Human Polkappa most frequently incorporated A opposite a template abasic site. Efficient further extension required T as the next template base, and was mediated mainly by a one-nucleotide deletion mechanism. Human Polkappa was able to bypass an acetylaminofluorene-modified G in DNA, incorporating either C or T, and less efficiently A opposite the lesion. Furthermore, human Polkappa effectively bypassed a template (-)-trans-anti-benzo[a]pyrene-N:(2)-dG lesion in an error-free manner by incorporating a C opposite the bulky adduct. In contrast, human Polkappa was unable to bypass a template TT dimer or a TT (6-4) photoproduct, two of the major UV lesions. These results suggest that Polkappa plays an important role in both error-free and error-prone lesion bypass in humans.

Skin cancer is the most prevalent form of cancer, and its incidence has been steadily rising over the past 20 years.1 Squamous cell and basal cell carcinomas will affect about one out of four Americans in their lifetime, and the most serious form, malignant melanoma, is expected to affect one in 100 by the year 2000.2 Evidence that sunlight causes skin cancer by damaging the DNA comes from a rare genetic disease, xeroderma pigmentosum, which is characterized by defects in the ability of cells to repair DNA photodamage.3•4 Patients suffering from this disease are extremely sensitive to sunlight and have an approximately 1000-fold higher chance of developing skin cancer. Insight into the type of DNA photodamage responsible for inducing skin cancer comes from the analysis of the mutations found in the p53 tumor suppressor gene of skin cancers. Unlike protooncogenes which require specific mutations to become activated, tumor suppressor genes can be inactivated by a much broader range of mutations and thus retain the signature of the carcinogen. When the p53 genes of basal and squamous cell carcinomas were sequenced, a large number of C -* T mutations at dipyrimidine sites were discovered as well as the tandem CC -*• TT mutation.5•6 These mutations are highly characteristic of UVB and UVC radiation,7 which causes both cis-syn dimers and (6-4) products to form at these sites8 (Figure 1).

It is well known that exposure to UV induces DNA damage, which is the first step in mutagenesis and a major cause of skin cancer. Among a variety of photoproducts, cyclobutane-type pyrimidine photodimers (CPD) are the most abundant primary lesion. Despite its biological importance, the precise relationship between the structure and properties of DNA containing CPD has remained to be elucidated. Here, we report the free (unbound) crystal structure of duplex DNA containing a CPD lesion at a resolution of 2.0 Å. Our crystal structure shows that the overall helical axis bends Ϸ30°t oward the major groove and unwinds Ϸ9°, in remarkable agreement with some previous theoretical and experimental studies. There are also significant differences in local structure compared with standard B-DNA, including pinching of the minor groove at the 3 side of the CPD lesion, a severe change of the base pair parameter in the 5 side, and serious widening of both minor and major groves both 3 and 5 of the CPD. Overall, the structure of the damaged DNA differs from undamaged DNA to an extent that DNA repair proteins may recognize this conformation, and the various components of the replicational and transcriptional machinery may be interfered with due to the perturbed local and global structure. The cis-syn pyrimidine dimer (cyclobutane-type pyrimidine photodimer, CPD) is the major photoproduct induced by UV light present in sunlight (1) and is one of the principal causes of skin cancer (2). Evidence for the formation of the thymine dimer in DNA was first obtained Ͼ40 years ago (3, 4) and a few years later for cytosine-containing CPDs (5). Because of the mutagenic and protein-DNA-disrupting properties of CPDs, many organisms have evolved enzymes to specifically recognize and repair cis-syn dimers, such as Escherichia coli photolyase (6, 7), T4 endoV (8), as well as general repair enzymes such as E. coli uvrABC (9) and human excinuclease (10). Additionally, dimers are efficiently repaired in transcription-coupled repair by virtue of their ability to block synthesis by RNA polymerases (11,12). CPDs also block DNA replication (13) and are efficiently bypassed in a nonmutagenic manner by DNA damage bypass polymerases such as E. coli pol V (14) and the recently discovered yeast (15) and human polymerase (16,17). There is also evidence that the mismatch repair system can recognize mismatches opposite thymine dimers (18). Understanding the mechanism by which these proteins recognize and process CPDs will be greatly aided by a structure for CPD-containing DNA.The efficiency of damage repair is likely to depend on the extent to which those changes alter the structure of the DNA, hence making it recognizable for the repair enzymes involved. It has been suggested that the binding affinities of the repair enzymes for the CPD-containing DNA depend on the degree of DNA unwinding or kinking caused by those lesions (19,20). The first evidence that CPD formation causes large alterations in the structure of DNA is offered by circular dichroism studies and...

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DNA polymerase (Pol, xeroderma pigmentosum variant, or Rad30) plays an important role in an errorfree response to unrepaired UV damage during replication. It faithfully synthesizes DNA opposite a thyminethymine cis-syn-cyclobutane dimer. We have purified the yeast Pol and studied its lesion bypass activity in vitro with various types of DNA damage. The yeast Pol lacked a nuclease or a proofreading activity. It efficiently bypassed 8-oxoguanine, incorporating C, A, and G opposite the lesion with a relative efficiency of ϳ100: 56:14, respectively. The yeast Pol efficiently incorporated a C opposite an acetylaminofluorene-modified G, and efficiently inserted a G or less frequently an A opposite an apurinic/apyrimidinic (AP) site but was unable to extend the DNA synthesis further in both cases. However, some continued DNA synthesis was observed in the presence of the yeast Pol following the Pol action opposite an AP site, achieving true lesion bypass. In contrast, the yeast Pol␣ was able to bypass efficiently a template AP site, predominantly incorporating an A residue opposite the lesion. These results suggest that other than UV damage, Pol may also play a role in bypassing additional DNA lesions, some of which can be error-prone.