Complete genome sequence of Pseudomonas aeruginosa PAO1, an opportunistic pathogen

Pseudomonas aeruginosa is a ubiquitous environmental bacterium that is one of the top three causes of opportunistic human infections. A major factor in its prominence as a pathogen is its intrinsic resistance to antibiotics and disinfectants. Here we report the complete sequence of P. aeruginosa strain PAO1. At 6.3 million base pairs, this is the largest bacterial genome sequenced, and the sequence provides insights into the basis of the versatility and intrinsic drug resistance of P. aeruginosa. Consistent with its larger genome size and environmental adaptability, P. aeruginosa contains the highest proportion of regulatory genes observed for a bacterial genome and a large number of genes involved in the catabolism, transport and efflux of organic compounds as well as four potential chemotaxis systems. We propose that the size and complexity of the P. aeruginosa genome reflect an evolutionary adaptation permitting it to thrive in diverse environments and resist the effects of a variety of antimicrobial substances.

A small-molecule nitroimidazopyran drug candidate for the treatment of tuberculosis

Mycobacterium tuberculosis, which causes tuberculosis, is the greatest single infectious cause of mortality worldwide, killing roughly two million people annually. Estimates indicate that one-third of the world population is infected with latent M. tuberculosis. The synergy between tuberculosis and the AIDS epidemic, and the surge of multidrug-resistant clinical isolates of M. tuberculosis have reaffirmed tuberculosis as a primary public health threat. However, new antitubercular drugs with new mechanisms of action have not been developed in over thirty years. Here we report a series of compounds containing a nitroimidazopyran nucleus that possess antitubercular activity. After activation by a mechanism dependent on M. tuberculosis F420 cofactor, nitroimidazopyrans inhibited the synthesis of protein and cell wall lipid. In contrast to current antitubercular drugs, nitroimidazopyrans exhibited bactericidal activity against both replicating and static M. tuberculosis. Lead compound PA-824 showed potent bactericidal activity against multidrugresistant M. tuberculosis and promising oral activity in animal infection models. We conclude that nitroimidazopyrans offer the practical qualities of a small molecule with the potential for the treatment of tuberculosis.

A viral gene that activates lytic cycle expression of Kaposi's sarcoma-associated herpesvirus

Herpesviruses exist in two states, latency and a lytic productive cycle. Here we identify an immediate-early gene encoded by Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus eight (HHV8) that activates lytic cycle gene expression from the latent viral genome. The gene is a homologue of Rta, a transcriptional activator encoded by Epstein-Barr virus (EBV). KSHV/Rta activated KSHV early lytic genes, including virus-encoded interleukin 6 and polyadenylated nuclear RNA, and a late gene, small viral capsid antigen. In cells dually infected with Epstein-Barr virus and KSHV, each Rta activated only autologous lytic cycle genes. Expression of viral cytokines under control of the KSHV/Rta gene is likely to contribute to the pathogenesis of KSHV-associated diseases.

AML1-ETO expression is directly involved in the development of acute myeloid leukemia in the presence of additional mutations

The t(8;21) is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). The translocation, which involves the AML1 gene on chromosome 21 and the ETO gene on chromosome 8, generates an AML1-ETO fusion transcription factor. To examine the effect of the AML1-ETO fusion protein on leukemogenesis, we made transgenic mice in which expression of AML1-ETO is under the control of the human MRP8 promoter (hMRP8-AML1-ETO). AML1-ETO is specifically expressed in myeloid cells, including common myeloid progenitors of hMRP8-AML1-ETO transgenic mice. The transgenic mice were healthy during their life spans, suggesting that AML1-ETO alone is not sufficient for leukemogenesis. However, after treatment of newborn hMRP8-AML1-ETO transgenic mice and their wild-type littermates with a strong DNA-alkylating mutagen, N-ethyl-N-nitrosourea, 55% of transgenic mice developed AML and the other 45% of transgenic mice and all of the wild-type littermates developed acute T lymphoblastic leukemia. Our results provide direct evidence that AML1-ETO is critical for causing myeloid leukemia, but one or more additional mutations are required for leukemogenesis. The hMRP8-AML1-ETO-transgenic mice provide an excellent model that can be used to isolate additional genetic events and to further understand the molecular pathogenesis of AML1-ETO-related leukemia.

Effect of low- and high-frequency TENS on Met-enkephalin-Arg-Phe and dynorphin A immunoreactivity in human lumbar CSF

Transcutaneous nerve stimulation (TENS) treatment was given for 30 min to 37 patients divided into 3 groups of 10 patients and 1 group of 7 patients. Two groups received low-frequency (2 Hz) and the other 2 groups high-frequency (100 Hz) stimulation. A diagnostic lumbar cerebrospinal fluid (CSF) sample was obtained immediately before and after stimulation. The CSF samples were subjected to analysis of immunoreactive (ir) opioid peptides, Met-enkephalin-Arg-Phe (MEAP) from preproenkephalin and dynorphin A (Dyn A) from preprodynorphin, respectively. Low frequency TENS applied on the hand and the leg resulted in a marked increase (367%, P less than 0.05) of ir-MEAP but not ir-Dyn A, whereas high-frequency (100 Hz) TENS produced a 49% increase in ir-Dyn A (P less than 0.01) but not ir-MEAP. This is the first report in humans that 2 Hz and 100 Hz peripheral stimulation induces differential release of peptides from preproenkephalin and preprodynorphin, respectively.

Predicting function: from genes to genomes and back

Predicting function from sequence using computational tools is a highly complicated procedure that is generally done for each gene individually. This review focuses on the added value that is provided by completely sequenced genomes in function prediction. Various levels of sequence annotation and function prediction are discussed, ranging from genomic sequence to that of complex cellular processes. Protein function is currently best described in the context of molecular interactions. In the near future it will be possible to predict protein function in the context of higher order processes such as the regulation of gene expression, metabolic pathways and signalling cascades. The analysis of such higher levels of function description uses, besides the information from completely sequenced genomes, also the additional information from proteomics and expression data. The final goal will be to elucidate the mapping between genotype and phenotype.

Non-invasive prenatal testing for trisomies 21, 18 and 13: clinical experience from 146,958 pregnancies

OBJECTIVES

To report the clinical performance of massively parallel sequencing-based non-invasive prenatal testing (NIPT) in detecting trisomies 21, 18 and 13 in over 140,000 clinical samples and to compare its performance in low-risk and high-risk pregnancies.

METHODS

Between 1 January 2012 and 31 August 2013, 147,314 NIPT requests to screen for fetal trisomies 21, 18 and 13 using low-coverage whole-genome sequencing of plasma cell-free DNA were received. The results were validated by karyotyping or follow-up of clinical outcomes.

RESULTS

NIPT was performed and results obtained in 146,958 samples, for which outcome data were available in 112,669 (76.7%). Repeat blood sampling was required in 3213 cases and 145 had test failure. Aneuploidy was confirmed in 720/781 cases positive for trisomy 21, 167/218 cases positive for trisomy 18 and 22/67 cases positive for trisomy 13 on NIPT. Nine false negatives were identified, including six cases of trisomy 21 and three of trisomy 18. The overall sensitivity of NIPT was 99.17%, 98.24% and 100% for trisomies 21, 18 and 13, respectively, and specificity was 99.95%, 99.95% and 99.96% for trisomies 21, 18 and 13, respectively. There was no significant difference in test performance between the 72,382 high-risk and 40,287 low-risk subjects (sensitivity, 99.21% vs. 98.97% (P = 0.82); specificity, 99.95% vs. 99.95% (P = 0.98)). The major factors contributing to false-positive and false-negative NIPT results were maternal copy number variant and fetal/placental mosaicism, but fetal fraction had no effect.

CONCLUSIONS

Using a stringent protocol, the good performance of NIPT shown by early validation studies can be maintained in large clinical samples. This technique can provide equally high sensitivity and specificity in screening for trisomy 21 in a low-risk, as compared to high-risk, population.

The 16-kDa alpha-crystallin (Acr) protein of Mycobacterium tuberculosis is required for growth in macrophages

Although the 16-kDa alpha-crystallin homologue of Mycobacterium tuberculosis (MTB) is the dominant protein produced by stationary phase cultures in vitro, it is undetectable in logarithmically growing cultures. By growing bacilli at defined oxygen concentrations, acr transcription was shown to be strongly induced by mildly hypoxic conditions. Acr expression also was found to be induced during the course of in vitro infection of macrophages. The acr gene was replaced with a hygromycin resistance cassette by allelic exchange in MTB H37Rv. The resulting Deltaacr::hpt strain was shown to be equivalent to wild-type H37Rv in in vitro growth rate and infectivity but was significantly impaired for growth in both mouse bone marrow derived macrophages and THP-1 cells. In addition to its proposed role in maintenance of long-term viability during latent, asymptomatic infections, these results establish a role for the Acr protein in replication during initial MTB infection.

Stationary phase-associated protein expression in Mycobacterium tuberculosis: function of the mycobacterial alpha-crystallin homolog

The majority of active tuberculosis cases arise as a result of reactivation of latent organisms which are quiescent within the host. The ability of mycobacteria to survive extended periods without active replication is a complex process whose details await elucidation. We used two-dimensional gel electrophoresis to examine both steady-state protein composition and time-dependent protein synthetic profiles in aging cultures of virulent Mycobacterium tuberculosis. At least seven proteins were maximally synthesized 1 to 2 weeks following the end of log-phase growth. One of these proteins accumulated to become a predominant stationary-phase protein. N-terminal amino acid sequencing and immunoreactivity identified this protein as the 16-kDa alpha-crystallin-like small heat shock protein. The gene for this protein was shown to be limited to the slowly growing M. tuberculosis complex of organisms as assessed by Southern blotting. Overexpression of this protein in wild-type M. tuberculosis resulted in a slower decline in viability following the end of log-phase growth. Accumulation of this protein was observed in log-phase cultures following a shift to oxygen-limiting conditions but not by other external stimuli. The protein was purified to homogeneity from overexpressing M. smegmatis in two steps and shown to have a significant ability to suppress the thermal denaturation of alcohol dehydrogenase. Collectively, these results suggest that the mycobacterial alpha-crystallin protein may play a role in enhancing long-term protein stability and therefore long-term survival of M. tuberculosis.

Nucleotide and deduced amino acid sequences for the four variable domains of the major outer membrane proteins of the 15 Chlamydia trachomatis serovars

The amino acid sequences of major outer membrane proteins (MOMPs) from Chlamydia trachomatis serovars A, B, C, L1, and L2 are predominantly conserved but have four variable domains (VDs) in which major neutralizing and serotyping antigenic determinants are located. Because these MOMP VDs are primarily responsible for antigenic differences between serovars and are associated with important immunological and biological properties, we undertook studies focused on defining these sequences within the MOMPs of all 15 C. trachomatis serovars. We used oligonucleotide primer extension sequencing of MOMP mRNA to determine the nucleotide and deduced amino acid sequences of the four MOMP VDs of the 15 C. trachomatis serovars. Comparative amino acid sequence homologies of all four domains separated the serovars into three groups: group 1, serovars B, Ba, D, E, L1, and L2; group 2, serovars G and F; and group 3, serovars A, C, H, I, J, K, and L3. Hydrophilicity and charge values for each domain were determined. The MOMP VDs of given serovars with the greatest total hydrophilicity and charge values were found to be the location of antigenic determinants recognized by MOMP-specific monoclonal antibodies. These findings should be useful for predicting MOMP antigenic determinants and testing the antigenic properties of these VDs by using synthetic peptides corresponding to each MOMP VD. The potential usefulness of the VD sequence information is discussed in relation to the development of defined synthetic peptides and oligonucleotides that may be used to develop new serological and diagnostic assays for C. trachomatis infections.

Identification of the immediate-early transcripts of Kaposi's sarcoma-associated herpesvirus

In the immediate-early phase of reactivation or primary infection, herpesviruses express a small number of genes without requiring prior viral protein synthesis. Immediate-early genes usually encode regulatory proteins critical for the viral life cycle. Kaposi's sarcoma-associated herpesvirus (KSHV) gene transcription in the immediate-early stage of viral reactivation was examined by using a chemical induction combined with a gene expression screening method. RNA transcripts from at least four KSHV genomic loci accumulate when latently infected B-lymphoma cells are induced for reactivation in the presence of an inhibitor of protein synthesis (cycloheximide) and thus represent immediate-early class transcripts. Among them, a 3.6-kb mRNA encodes three putative open reading frames (ORFs), namely, ORF50, K8, and K8.2. ORF50 is a homologue of Rta, a transcription activator encoded by Epstein-Barr virus (EBV). The K8 gene codes for a 237-amino-acid protein with a basic-leucine zipper domain near its C terminus and an acidic domain near its N terminus and which closely resembles the ZEBRA protein of EBV and Jun/Fos family proteins. Other immediate-early mRNAs of KSHV include a 1. 7-kb mRNA encoding ORF45, a 2.0-kb mRNA encoding ORF K4.2, and a 4. 5-kb mRNA. Functional roles of products of these KSHV immediate-early transcripts remain to be studied.

Differences in genotypes of Helicobacter pylori from different human populations

DNA motifs at several informative loci in more than 500 strains of Helicobacter pylori from five continents were studied by PCR and sequencing to gain insights into the evolution of this gastric pathogen. Five types of deletion, insertion, and substitution motifs were found at the right end of the H. pylori cag pathogenicity island. Of the three most common motifs, type I predominated in Spaniards, native Peruvians, and Guatemalan Ladinos (mixed Amerindian-European ancestry) and also in native Africans and U.S. residents; type II predominated among Japanese and Chinese; and type III predominated in Indians from Calcutta. Sequences in the cagA gene and in vacAm1 type alleles of the vacuolating cytotoxin gene (vacA) of strains from native Peruvians were also more like those from Spaniards than those from Asians. These indications of relatedness of Latin American and Spanish strains, despite the closer genetic relatedness of Amerindian and Asian people themselves, lead us to suggest that H. pylori may have been brought to the New World by European conquerors and colonists about 500 years ago. This thinking, in turn, suggests that H. pylori infection might have become widespread in people quite recently in human evolution.

Enrichment of gene-coding sequences in maize by genome filtration

Approximately 80% of the maize genome comprises highly repetitive sequences interspersed with single-copy, gene-rich sequences, and standard genome sequencing strategies are not readily adaptable to this type of genome. Methodologies that enrich for genic sequences might more rapidly generate useful results from complex genomes. Equivalent numbers of clones from maize selected by techniques called methylation filtering and High C0t selection were sequenced to generate approximately 200,000 reads (approximately 132 megabases), which were assembled into contigs. Combination of the two techniques resulted in a sixfold reduction in the effective genome size and a fourfold increase in the gene identification rate in comparison to a nonenriched library.

DNA repair and recombination factor Rad51 is over-expressed in human pancreatic adenocarcinoma

Molecular processes that could contribute to differences in chemo- and radioresistance include variations in DNA repair mechanisms. In mammalian cells, the product of the rad51 gene mediates DNA repair via homologous recombination. We describe that in contrast to conventional monolayer cell systems Rad51 protein accumulates to high-levels in three-dimensional cell culture models as well as in orthotopic xeno-transplants of human pancreatic cancer cells. Strikingly, over-expression of wild-type Rad51 was also found in 66% of human pancreatic adenocarcinoma tissue specimens. Functional analysis revealed that Rad51 over-expression enhances survival of cells after induction of DNA double strand breaks. These data suggest that perturbations of Rad51 expression contribute to the malignant phenotype of pancreatic cancer. Oncogene (2000).

Disparate responses to oxidative stress in saprophytic and pathogenic mycobacteria

To persist in macrophages and in granulomatous caseous lesions, pathogenic mycobacteria must be equipped to withstand the action of toxic oxygen metabolites. In Gram-negative bacteria, the OxyR protein is a critical component of the oxidative stress response. OxyR is both a sensor of reactive oxygen species and a transcriptional activator, inducing expression of detoxifying enzymes such as catalase/hydroperoxidase and alkyl hydroperoxidase. We have characterized the responses of various mycobacteria to hydrogen peroxide both phenotypically and at the levels of gene and protein expression. Only the saprophytic Mycobacterium smegmatis induced a protective oxidative stress response analogous to the OxyR response of Gram-negative bacteria. Under similar conditions, the pathogenic mycobacteria exhibited a limited, nonprotective response, which in the case of Mycobacterium tuberculosis was restricted to induction of a single protein, KatG. We have also isolated DNA sequences homologous to oxyR and ahpC from M. tuberculosis and Mycobacterium avium. While the M. avium oxyR appears intact, the oxyR homologue of M. tuberculosis contains numerous deletions and frameshifts and is probably nonfunctional. Apparently the response of pathogenic mycobacteria to oxidative stress differs significantly from the inducible OxyR response of other bacteria.

Focal adhesion kinase (pp125FAK) cleavage and regulation by calpain

Focal adhesion kinase (125 kDa form; pp125FAK) is a widely expressed non-receptor tyrosine kinase that is implicated in integrin-mediated signal transduction. We have identified a novel means of pp 125FAK regulation in human platelets, in which this kinase undergoes sequential proteolytic modification from the native 125 kDa form to 90, 45 and 40 kDa fragments in thrombin-, collagen- and ionophore A23187-stimulated platelets. The proteolysis of pp125FAK was prevented by pretreating platelets with the calpain inhibitors calpeptin or calpain inhibitor-1, and was reproduced in vitro by incubating immunoprecipitated pp125FAK with purified calpain. Proteolysis of pp125FAK resulted in a dramatic reduction in its autokinase activity and led to its dissociation from the cytoskeletal fraction of platelets. These studies define a novel signal-terminating role for calpain, wherein proteolytic modification of pp125FAK attenuates its autokinase activity and induces its subcellular relocation within the cell.

Identification of a gene involved in the biosynthesis of cyclopropanated mycolic acids in Mycobacterium tuberculosis

Mycolic acids represent a major constituent of the mycobacterial cell wall complex, which provides the first line of defense against potentially lethal environmental conditions. Slow-growing pathogenic mycobacteria such as Mycobacterium tuberculosis modify their mycolic acids by cyclopropanation, whereas fast-growing saprophytic species such as Mycobacterium smegmatis do not, suggesting that this modification may be associated with an increase in oxidative stress experienced by the slow-growing species. We have demonstrated the transformation of the distal cis double bond in the major mycolic acid of M. smegmatis to a cis-cyclopropane ring upon introduction of cosmid DNA from M. tuberculosis. This activity was localized to a single gene (cma1) encoding a protein that was 34% identical to the cyclopropane fatty acid synthase from Escherichia coli. Adjacent regions of the DNA sequence encode open reading frames that display homology to other fatty acid biosynthetic enzymes, indicating that some of the genes required for mycolic acid biosynthesis may be clustered in this region. M. smegmatis overexpressing the cma1 gene product significantly resist killing by hydrogen peroxide, suggesting that this modification may be an important adaptation of slow-growing mycobacteria to oxidative stress.

The biosynthesis of cyclopropanated mycolic acids in Mycobacterium tuberculosis. Identification and functional analysis of CMAS-2

The major mycolic acid produced by Mycobacterium tuberculosis contains two cis-cyclopropanes in the meromycolate chain. The gene whose product cyclopropanates the proximal double bond was cloned by homology to a putative cyclopropane synthase identified from the Mycobacterium leprae genome sequencing project. This gene, named cma2, was sequenced and found to be 52% identical to cma1 (which cyclopropanates the distal double bond) and 73% identical to the gene from M. leprae. Both cma genes were found to be restricted in distribution to pathogenic species of mycobacteria. Expression of cma2 in Mycobacterium smegmatis resulted in the cyclopropanation of the proximal double bond in the alpha 1 series of mycolic acids. Coexpression of both cyclopropane synthases resulted in cyclopropanation of both centers, producing a molecule structurally similar to the M. tuberculosis alpha-dicyclopropyl mycolates. Differential scanning calorimetry of purified cell walls and mycolic acids demonstrated that cyclopropanation of the proximal position raised the observed transition temperature by 3 degrees C. These results suggest that cyclopropanation contributes to the structural integrity of the cell wall complex.

The effect of oxygenated mycolic acid composition on cell wall function and macrophage growth in Mycobacterium tuberculosis

There are three major structural classes of mycolic acids in the cell envelope of Mycobacterium tuberculosis (MTB): alpha-, methoxy- and ketomycolate. The two oxygen-containing classes are biosynthetically related through a common alpha-methyl hydroxymycolate intermediate. BCG strains that fail to produce methoxymycolate and instead produce only keto- and alpha-mycolic acids show apparent defects in the O-methyltransferase MMAS-3. Overproduction of MMAS-3 from MTB resulted in a complete replacement of ketomycolate by methoxymycolate in both BCG and MTB. In vitro growth of these recombinant strains lacking ketomycolate was impaired at reduced temperatures but appeared to be normal at 37 degrees C. Glucose uptake was significantly decreased in such strains, but uptake of chenodeoxycholate and glycine was unaffected. Although sensitivity to INH remained unchanged, these cells were found to be hypersensitive to ampicillin and rifampicin. Infectivity of BCG and H37Rv wild type or MMAS-3 overproducers in THP-1 cells was somewhat affected, but the ability of the strains lacking ketomycolate to grow within this macrophage-like cell line was severely compromised. In vivo labelling of mycolic acids during growth of H37Rv within THP-1 cells revealed a substantial increase in ketomycolate and alphamycolate synthesized by intracellularly grown mycobacteria. These results establish a critical role for mycolate composition in proper cell wall function during the growth of MTB in vivo.

Fluorodeoxyglucose-positron-emission tomography, single-photon emission tomography, and structural MR imaging for prediction of rapid conversion to Alzheimer disease in patients with mild cognitive impairment: a meta-analysis

BACKGROUND AND PURPOSE

Patients with mild cognitive impairment (MCI) are at risk for developing Alzheimer disease (AD). To diagnose AD at an early stage, one must develop highly specific and sensitive tools to identify it among at-risk subjects. The purpose of this study was to evaluate and compare the ability of fluorodeoxyglucose-positron-emission tomography (FDG-PET), single-photon emission tomography (SPECT), and structural MR imaging to predict conversion to AD in patients with MCI.

MATERIALS AND METHODS

Relevant studies were identified with MEDLINE from January 1990 to April 2008. Meta-analysis and meta-regression were done on the diagnostic performance data for each technique from eligible studies. We estimated and compared the weighted summary sensitivities, specificities, likelihood ratios (LRs), and summary receiver operating characteristic curves of each imaging technique.

RESULTS

Twenty-four eligible studies were included, with a total of 1112 patients. FDG-PET performed statistically better in LR+ and odds ratio (OR), whereas no statistical difference was found in pooled sensitivity, specificity, and LR- for each technique. No statistical difference was confirmed between SPECT and MR imaging. The Q* index estimates for FDG-PET, SPECT, and structural MR imaging were respectively 0.86, 0.75, and 0.76. In meta-regression, statistical significance was found only between technique and log OR, with a regression coefficient of -0.575.

CONCLUSIONS

This meta-analysis showed that FDG-PET performs slightly better than SPECT and structural MR imaging in the prediction of conversion to AD in patients with MCI; parallel performance was found between SPECT and MR imaging.

A common mechanism for the biosynthesis of methoxy and cyclopropyl mycolic acids in Mycobacterium tuberculosis

Mycobacterium tuberculosis produces three classes of mycolic acids that differ primarily in the presence and nature of oxygen-containing substituents in the distal portion of the meromycolate branch. The methoxymycolate series has a methoxy group adjacent to a methyl branch, in addition to a cyclopropane in the proximal position. Using the gene for the enzyme that introduces the distal cyclopropane (cma1) as a probe, we have cloned and sequenced a cluster of genes coding for four highly homologous methyl transferases (mma1-4). When introduced into Mycobacterium smegmatis, this gene cluster conferred the ability to synthesize methoxymycolates. By determining the structure of the mycolic acids produced following expression of each of these genes individually and in combination, we have elucidated the biosynthetic steps responsible for the production of the major series of methoxymycolates. The mma4 gene product (MMAS-4) catalyzes an unusual S-adenosyl-L-methionine-dependent transformation of the distal cis-olefin into a secondary alcohol with an adjacent methyl branch. MMAS-3 O-methylates this secondary alcohol to form the corresponding methyl ether, and MMAS-2 introduces a cis-cyclopropane in the proximal position of the methoxy series. The similarity of these reactions and the enzymes that catalyze them suggests that some of the structural diversity of mycolic acids results from different chemical fates of a common cationic intermediate, which in turn results from methyl group addition to an olefinic mycolate precursor.

Targeted cleavage of mRNA by human RNase P

Ribonuclease P from Escherichia coli can cleave RNAs in simple, hydrogen-bonded complexes of two oligoribonucleotides that resemble the aminoacyl stem and 5' leader sequence of tRNA precursors. RNase P from human (HeLa) cells cannot catalyze the cleavage in vitro of the 5'-proximal oligoribonucleotide that contains the leader sequence in such simple complexes but can do so when the 3'-proximal oligoribonucleotide (external guide sequence) is altered to resemble three-quarters of a tRNA molecule. In such a complex, the efficiency of cleavage of the mRNA for chloramphenicol acetyltransferase, as the 5'-proximal oligoribonucleotide, depends on the structural details of the external guide sequence and on the choice of target site within the mRNA. The presence of the appropriately designed external guide sequence in cells in tissue culture reduces chloramphenicol acetyltransferase activity and the level of the corresponding intact mRNA in the cells. Thus, it appears that the use of such external guide sequences may provide a general technique for gene inactivation.