Inverse method for the determination of a mathematical expression for the anisotropy of the solid-liquid interfacial energy in Al-Zn-Si alloys

An expression for the anisotropy of the solid-liquid interfacial energy has been determined experimentally by an inverse method for the Al-43.4 wt%Zn-1.6 wt%Si system. Assuming that dendrite growth directions correspond to the minima of the surface stiffness, the anisotropy of the solid-liquid interfacial energy could be described by minimizing the errors between the calculated minima of a parametric interface stiffness function and experimentally measured growth directions of dendrites in thin coatings. In order to adequately describe the interfacial energy, it is found that a cubic harmonic expansion up to the third order is necessary to obtain the minima of interface stiffness along directions that depart from <100> or <110>. Best agreement with observed growth directions is obtained for first, second, and third harmonic coefficients (epsilon1, epsilon2, and epsilon3, respectively) satisfying the following relationships: epsilon2/epsilon1 = -0.188; epsilon3/epsilon1 = -0.00776. The corresponding interface stiffness function shows 24 minima lying along directions between <100> and <110>. The minima are located at 28.5 degrees from <100> and only 5.1 degrees from <320>, which was the growth direction suggested by Sémoroz for this alloy [A. Sémoroz, Y. Durandet, and M. Rappaz, Acta Mater. 49, 529 (2001).]. It was also found that the strength of the effective in-plane anisotropy is directly reflected by the morphology of the dendritic microstructure.

Phosphodiesterase 11: a brief review of structure, expression and function

Phosphodiesterase 11 (PDE11) is the latest isoform of the phosphodiesterase family to be identified. Interest in PDE11 has increased recently because tadalafil, an oral phosphodiesterase 5 inhibitor, cross reacts with PDE11. The function of PDE11 remains largely unknown, but growing evidence points to a possible role in male reproduction. The published literature on PDE11 structure, function and expression is reviewed.

Predicting hypogonadism in men based upon age, presence of erectile dysfunction, and depression

Hypogonadism, a disorder associated with aging, can cause significant morbidity. As clinical manifestations of hypogonadism can be subtle, the challenge and the burden of diagnosis remain the responsibility of the clinician. Four different analytic methods were used to predict hypogonadism in men based upon age, the presence of erectile dysfunction (ED) and depression. 218 men were classified by age, serum testosterone level, the presence of ED and depression. Depression was determined by the Center for Epidemiologic Studies Depression Scale (CES-D). ED was assessed by the Sexual Health Inventory for Men (SHIM). Hypogonadism was defined as a serum testosterone level <300 ng/dl. An artificial neural network (ANN) was programmed and trained to predict hypogonadism based upon age, SHIM, and CES-D scores. Subject data was randomly partitioned into a training set of 148 (67.9%) and a test set of 70 (32.1%). The ANN processed the test set only after the training was complete. The discrete predicted binary output was set to (0) if testosterone level was <300 ng/dl or (1) if >300 ng/dl. The data was also analyzed by standard logistic regression (LR), linear and quadratic discriminant function analysis (LDFA and QDFA, respectively). Reverse regression (RR) analysis evaluated the statistical significance of each risk factor. The ANN can accurately predict hypogonadism in men based upon age, the presence of ED, and depression (receiver-operating characteristic=0.725). A four hidden node network was found to have the highest accuracy. RR revealed the depression index score to be most significant variable (P=0.0019), followed by SHIM score (P=0.00602), and then by age (P=0.015). Hypogonadism can be predicated by an ANN using the input factors of age, ED, and depression. This model can help clinicians assess the need for endocrinologic evaluation in men.

Human chorionic gonadotropin free beta-subunit in the human seminal plasma: a new marker for spermatogenesis?

In the past 20 years, several factors were detected in the human seminal plasma and proposed as markers for spermatogenesis. Human chorionic gonadotropin (hCG) and its beta-subunit were therefore investigated, and their seminal levels were found to be higher than those detected in the serum and to correlate with sperm parameters.

OBJECTIVE

We designed a retrospective study to determine the suitability of hCG free beta-subunit concentration in the seminal plasma of fertile and infertile male patients as marker of spermatogenesis.

STUDY DESIGN

A total of 79 infertile male patients were divided into four groups by their semen analysis results (group 1 [n=8]: azoospermia; group 2 [n=21]: severe oligozoospermia; group 3 [n=40]: oligoasthenospermia (OAS); group 4 [n=10]: asthenospermia) and 10 healthy volunteers of proven fertility were evaluated.

RESULTS

The hCG free beta-subunit levels in the seminal plasma were found to be significantly higher (P<0.0001) in the control group in respect to those assayed in the infertile patients and showed a correlation with sperm count (r=0.5) and total motile sperm density (r=0.5). Twenty-five patients were on treatment with oral Mesterolone (100mg daily) plus Tamoxifen (20mg daily) for 3-6 months. Apart from a significant improvement (P<0.05) in sperm morphology, no significant changes in sperm count and motility were observed after the treatment in all the patients. In the seminal plasma of 10 patients who showed a significant increase in sperm count, hCG free beta-subunit levels were found to be significantly higher compared to those detected in the remaining patients (P<0.01). In all patients, these levels remained unchanged after the treatment.

CONCLUSIONS

The evidence regarding the positive correlation between hCG free beta-subunit levels in the seminal plasma and sperm concentration is consistent with the previous results regarding hCG levels. A previous study demonstrated that testosterone levels in seminal plasma correlated with sperm concentrations; from the same evidence regarding hCG we hypothesize that seminal plasma testosterone and hCG levels are correlated. Thus, hCG may play a paracrine role in the intratesticular regulation of testosterone secretion.

Microsatellite instability and defects in mismatch repair proteins: a new aetiology for Sertoli cell-only syndrome

Microsatellite instability is characteristic of certain types of cancer, and is present in rodents lacking specific DNA mismatch repair proteins. These azoospermic mice exhibit spermatogenic defects similar to some human testicular failure patients. Therefore, we hypothesized that microsatellite instability due to deficiencies in mismatch repair genes might be an unrecognized aetiology of human testicular failure. Because these azoospermic patients are candidates for testicular sperm extraction and ICSI, transmission of mismatch repair defects to the offspring is possible. Seven microsatellite loci were analysed for instability in specimens from 41 testicular failure patients and 20 controls. Blood and testicular DNA were extracted from patient and control specimens, and amplified by PCR targeting seven microsatellite loci. DNA fragment length was analysed with an ABI Prism 310 Genotyping Machine and GeneScan software. Immunohistochemistry was performed on paraffinized testis biopsy sections and cultured testicular fibroblasts from each patient to determine if expression of the mismatch repair proteins hMSH2 and hMLH1 was normal in both somatic and germline cells. Results demonstrate that microsatellite instability and DNA mismatch repair protein defects are present in some azoospermic men, predominantly in Sertoli cell-only patients (P < 0.01 and P < 0.05 respectively). This provides evidence of a previously unrecognized aetiology of testicular failure that may be associated with cancer predisposition.

Neural computation in urology: an orientation

Neural computation is a field in which mathematical models are derived from algorithms based loosely on the physiological function of the biological neuron. This paper serves as an introduction to those that follow in this issue of Molecular Urology, which describe actual applications of neural computational modeling in the urologic domain. In this introductory paper, the history of computer technology and the foundations of neural computation are discussed. Methods of determining the accuracy of computation models are reviewed, and a statistical method of evaluating the significance of individual input features to the model's output, a process known as "feature extraction," is presented. Resources that provide free and commercial neural computational programs are cited for those readers interested in applying this technology to their own datasets, and a brief description of the author's neural computational programming environment is included. Finally, deployment of computational models via the Internet and various computer platforms is discussed.

The pathophysiology of varicocele-associated male infertility

Varicocele is the most commonly identifiable, surgically correctable lesion associated with male-factor infertility. Surgical correction of a varicocele, whether unilateral or bilateral, results in improvement not only in semen parameters but also in spontaneous and assisted pregnancy rates. Varicoceles seem to induce a number of changes in the testicular microenvironment. These alterations in temperature, hemodynamics, and reactive oxidative species and antioxidant concentrations have been demonstrated to produce deleterious effects on spermatogenesis. However, despite current knowledge in the pathophysiology of varicocele-associated male infertility, the exact mechanism--or mechanisms--by which varicoceles impair fertility remains elusive. This review examines scientific evidence regarding the pathophysiology of varicocele-associated male infertility.

In vitro fertilization outcomes after intracytoplasmic sperm injection with fresh or frozen-thawed testicular spermatozoa

OBJECTIVE

To compare the outcomes of intracytoplasmic sperm injection (ICSI) and in vitro fertilization (IVF) with fresh and cryopreserved testicular spermatozoa in patients with obstructive and nonobstructive azoospermia.

DESIGN

Retrospective analysis of consecutive ICSI cycles.

SETTING

Large urban reproductive medicine program.

PATIENT(S)

Twenty-nine patients with obstructive and nonobstructive azoospermia undergoing testicular sperm extraction for a total of 46 IVF-ICSI cycles (12 fresh, 34 frozen).

INTERVENTION(S)

Testicular sperm extraction, cryopreservation, and IVF-ICSI with fresh or frozen-thawed spermatozoa.

MAIN OUTCOME MEASURE(S)

Fertilization rates, embryo cleavage rates, embryo implantation rates, clinical pregnancy rates per cycle and per embryo transfer, and delivery and spontaneous abortion rates.

RESULT(S)

No statistically significant differences were noted in any of the parameters examined between IVF-ICSI cycles from fresh or frozen-thawed testicular spermatozoa. Fertilization rates were 56% with fresh vs. 61% with frozen-thawed testicular sperm, cleavage rates 92% vs. 95%, implantation rates 26% vs. 17%, clinical pregnancy rates per cycle 33% vs. 41%, and pregnancy rates per embryo transfer 33% vs. 45%, respectively. Delivery rates were 75% with fresh vs. 69.2% with frozen-thawed testicular sperm, and spontaneous abortion rates 25% and 30.8%, respectively.

CONCLUSION(S)

No differences were found in IVF-ICSI outcomes between cryopreserved and fresh testicular sperm. In addition, cryopreservation provides several advantages for the patients and reproductive team.

The tRNA N2,N2-dimethylguanosine-26 methyltransferase encoded by gene trm1 increases efficiency of suppression of an ochre codon in Schizosaccharomyces pombe

In the majority of eukaryotic tRNAs, the guanosine at position 26 is modified by a dimethyl group, but so far a function of this modification has not been detected. We isolated the Schizosaccharomyces pombe gene, trm1, encoding the tRNA N2, N2-dimethylguanosine-26 methyltransferase. Strains having the gene deleted completely lack N2,N2-dimethylguanosine. In strains carrying the weak ochre tRNA suppressor sup3-i, deletion of trm1 abolishes suppression indicating that the trm1 deletion acts as an antisuppressor mutation. The result suggests that in vivo N2, N2-dimethylguanosine-26 increases the capacity of the sup3-i serine tRNA to translate the UAA (ochre) codon.

Potency preservation following conformal radiotherapy for localized prostate cancer: impact of neoadjuvant androgen blockade, treatment technique, and patient-related factors

PURPOSE

Impotence is a familiar sequela of both definitive external-beam radiotherapy (EBRT) and radical prostatectomy for localized prostate cancer. Among surgical options, nerve-sparing radical prostatectomy (NSRP) offers the highest potency preservation rate of 70%. We report the change in potency over time in an EBRT-treated population, determine the significantly predisposing health and treatment factors affecting post-EBRT potency, and compare age- and stage-matched potency rates with those of NSRP-treated patients.

PATIENTS AND METHODS

Our results are from a retrospective study of 287 patients diagnosed with prostate cancer in clinical stages A to C and treated with conformal techniques to 6200 to 7380 cGy. Information regarding preradiotherapy potency, medical and surgical history, neoadjuvant antiandrogen use, and post-EBRT potency was documented for each patient. The median follow-up time was 34 months.

RESULTS

At months 1, 20, 40, and 60, actuarial potency rates were 96%, 75%, 59%, and 53%, respectively. Factors identified as significant predictors of post-EBRT impotence include pre-EBRT partial potency, diabetes, coronary artery disease, and anti-androgen medication usage. Among treatment factors, a trend toward potency preservation was noted for the six-field versus the four-field technique. Finally, age- and stage-matched comparisons of potency rates for our population and NSRP-treated patients were performed. For patients older than 70 years, 60.9% of EBRT patients and 32.9% of NSRP patients remained potent after treatment. Overall, EBRT patient potency preservation was 71.3%, versus 66.2% for NSRP patients.

DISCUSSION

Pre-EBRT partial potency, diabetes, coronary artery disease, and anti-androgen medication usage are significant predispositions to impotence in EBRT-treated prostate cancer patients. In comparing EBRT with NSRP for various age and stage groups, EBRT offers notably higher potency preservation rates than NSRP for patients older than 70 years.

Quality of cryopreserved testicular sperm in patients with obstructive and nonobstructive azoospermia

PURPOSE

Sperm retrieved by testicular sperm extraction is routinely used to attempt pregnancy by in vitro fertilization-intracytoplasmic sperm injection. We evaluated the efficacy of cryopreserving testicular sperm collected by testicular sperm extraction at diagnostic biopsy.

MATERIALS AND METHODS

A total of 73 men with obstructive and 42 with nonobstructive azoospermia underwent testicular sperm extraction at diagnostic biopsy. Sperm was retrieved and cryopreserved in all cases of obstruction and in 15 of nonobstructive azoospermia cases. Before freezing we determined sperm count, motility, morphology and viability, and after thawing we assessed sperm motility and viability. In 17 couples a total of 20 cycles of in vitro fertilization-intracytoplasmic sperm injection were performed and fertilization, cleavage and pregnancy rates were determined in cases of obstruction and nonobstruction.

RESULTS

Sperm count and morphology were lower in the testicular biopsies of men with nonobstructive versus obstructive azoospermia. Motility was low or absent in all testicular sperm extraction specimens. Importantly, pre-freeze (63%) and post-thaw (31%) viability was the same in both patient groups. After in vitro fertilization-intracytoplasmic sperm injection using frozen and thawed testicular sperm the fertilization, cleavage, implantation and clinical pregnancy rates were 60, 86, 16 and 50%, respectively. Using cryopreserved sperm we observed no differences in outcome of any in vitro fertilization-intracytoplasmic sperm injection procedure in patients with obstructive versus nonobstructive azoospermia.

CONCLUSIONS

Cryopreservation of testicular sperm provides enough good quality sperm after thawing to result in excellent in vitro fertilization-intracytoplasmic sperm injection outcomes. Cryopreservation does not adversely affect intracytoplasmic sperm injection outcomes, including pregnancy rate. Therefore, we recommend routine testicular sperm extraction and cryopreservation of sperm at testicular biopsy.

A Schizosaccharomyces pombe gene, ksg1, that shows structural homology to the human phosphoinositide-dependent protein kinase PDK1, is essential for growth, mating and sporulation

Fission yeast (Schizosaccharomyces pombe) requires inositol for growth, mating and sporulation. To define putative genes that are involved in the processing and transduction of the inositol signal, mutants that are temperature sensitive for growth and sporulation were selected on a medium containing non-limiting amounts of inositol. Two such mutants (ksg1-208 and ksg1-358) were analyzed, which are impaired in mating and sporulation at 30 degrees C and undergo growth arrest in the G2 phase of the cell cycle at 35 degrees C. The ksg1 gene was isolated by functional complementation. It maps on the left arm of chromosome II and encodes a putative 592-amino acid protein which exhibits good structural homology to a human 3-phosphoinositide-dependent protein kinase (PDK1) and its rat and Drosophila homologues. The two mutants have the same substitution at amino acid position 159: a glycine residue is replaced by glutamic acid. Deletion of the gene is lethal for haploid cells. We propose that ksg1 is involved in one or several phosphoinositide signalling processes that are responsible for control of the life cycle.

Expression of the ErbB family of receptors in ovarian cancer

OBJECTIVE

To characterize the relative levels of the ErbB family of receptors and their relationship to one another in ovarian cancer.

METHODS

A total of 17 serous cystadenocarcinomas were analyzed for epidermal growth factor receptor (EGF-R or ErbB-1) and ErbB-2, ErbB-3, and ErbB-4 receptor expression by Western blot analysis. Receptor levels were quantified by densitometry and expressed as relative densitometry units normalized to the level of alpha-tubulin. Linear regression analysis was used to analyze receptor group differences. A value of P < or = .05 was considered statistically significant.

RESULTS

All 17 tumors expressed detectable levels of EGF-R, ErbB-2, and ErbB-3, but ErbB-4 expression was not detected. EGF-R levels correlated with ErbB-2 (r = .70, P < .01) and ErbB-3 (r = .52, P < .05) levels. The highest correlation was obtained between the levels of ErbB-2 and ErbB-3 (r = 0.81, P < .001).

CONCLUSION

This study indicates an association between the levels of ErbB receptor family members in ovarian cancer. This association suggests that one or more coordinated regulatory mechanisms may be involved in determining their relative expression levels to one another.

Exogenous inositol and genes responsible for inositol transport are required for mating and sporulation in Shizosaccharomyces pombe

Fission yeast, Schizosaccharomyces pombe, is a natural inositol auxotroph. We show here that the amount of exogenous inositol added to the medium is critical for the control of its life cycle. Above growth-limiting concentrations inositol stimulates mating and sporulation in minimal medium. The effect of inositol is also observed on yeast-extract-medium plates. We selected a mutant, IM49, which mates and sporulates only poorly and show that it is defective in inositol transport. Its defect is in a gene (itr2) coding for a putative 12 membrane-spanning protein. The polypeptide contains the two sugar-transport motifs typical for hexose transporters and shows good homology to the two Saccharomyces cerevisiae inositol transporters. The itr2 gene is essential for cell growth and its mRNA level is repressed by glucose. Mutant IM49 is also complemented by a multicopy suppressor gene (itr1) which codes for a putative hexose transporter with unknown substrate specifity.

Cloning, expression analysis and chromosomal localization of the human nuclear receptor gene GCNF

Germ cell nuclear factor (GCNF) is an orphan member of the nuclear receptor gene superfamily. We report the cloning of a cDNA encoding a new variant of human GCNF from human testis and its expression analysis. Southern blot analysis of the human genomic DNA indicates that the GCNF gene is not closely related to other members within the nuclear receptor superfamily. Chromosomal localization of the GCNF gene shows that the gene is located on chromosome 9 at the locus q33-34.1. In situ hybridization analysis of GCNF expression in the testis shows that human GCNF is expressed exclusively in germ cells.

Characterization of germ cell-specific expression of the orphan nuclear receptor, germ cell nuclear factor

Nuclear receptors, such as those for androgens, estrogens, and progesterones, control many reproductive processes. Proteins with structures similar to these receptors, but for which ligands have not yet been identified, have been termed orphan nuclear receptors. One of these orphans, germ cell nuclear factor (GCNF), has been shown to be germ cell specific in the adult and, therefore, may also participate in the regulation of reproductive functions. In this paper, we examine more closely the expression patterns of GCNF in germ cells to begin to define spatio-temporal domains of its activity. In situ hybridization showed that GCNF messenger RNA (mRNA) is lacking in the testis of hypogonadal mutant mice, which lack developed spermatids, but is present in the wild-type testis. Thus, GCNF is, indeed, germ cell specific in the adult male. Quantitation of the specific in situ hybridization signal in wild-type testis reveals that GCNF mRNA is most abundant in stage VII round spermatids. Similarly, Northern analysis and specific in situ hybridization show that GCNF expression first occurs in testis of 20-day-old mice, when round spermatids first emerge. Therefore, in the male, GCNF expression occurs postmeiotically and may participate in the morphological changes of the maturing spermatids. In contrast, female expression of GCNF is shown in growing oocytes that have not completed the first meiotic division. Thus, GCNF in the female is expressed before the completion of meiosis. Finally, the nature of the two different mRNAs that hybridize to the GCNF complementary DNA was studied. Although both messages contain the DNA binding domain, only the larger message is recognized by a probe from the extreme 3' untranslated region. In situ hybridization with these differential probes demonstrates that both messages are present in growing oocytes. In addition, the coding region and portions of the 3' untranslated region of the GCNF complementary DNA are conserved in the rat.

Expression of epidermal growth factor and androgen receptors in ovarian cancer

Ovarian cancer is the second most common malignancy of the female reproductive tract. Approximately 50% of ovarian cancers have elevated levels of epidermal growth factor receptor (EGFR). This overexpression is correlated with a poor prognosis for patient survival. Ovarian cancers also express a number of sex steroid receptors. The androgen receptor (AR) is the predominant sex steroid receptor and is expressed in over 80% of ovarian cancers. We investigated whether a relationship exists between EGFR and AR in ovarian cancer. Sixty serous cystadenocarcinomas were analyzed for their relative levels of EGFR and AR by Western blot analysis. Data were analyzed by Student's t test and linear regression analysis for statistical significance. More than 98% of the tumors expressed detectable levels of EGFR, while 65% of the tumors expressed detectable levels of AR. The levels of EGFR (mean +/- SEM) were found to be significantly (P < 0.01) higher in AR+ (516 +/- 15) than in AR- (304 +/- 57) tumors. EGFR levels significantly correlated to AR levels (r = 0.49, P < 0.001). These results demonstrate an association between EGFR and AR levels in ovarian cancer. Whether this association represents a causal or a casual relationship remains to be determined.

Expression of a truncated epidermal growth factor receptor-like protein (TEGFR) in ovarian cancer

The epidermal growth factor receptor (EGFR) system has been implicated in the etiology of numerous cancers, including that of ovarian cancer. Elevated levels of EGFR are associated with poor patient prognosis. Moreover, a significant number of ovarian cancers express both the receptor and one of its ligands, suggesting an autocrine mechanism for autonomous tumor growth. Because of the implicated role of the EGFR system in neoplasia, a greater understanding of the factors involved in this system is necessary. We have recently characterized a truncated EGFR-like protein (TEGFR) in human placenta, and we now extend this investigation to ovarian cancer. We report that TEGFR is expressed in ovarian cancer and its level correlates to that of EGFR. Moreover, the level of TEGFR is reduced in metastatic compared to primary tumors. These results suggest that TEGFR may play a role in the EGFR system.

Computational tools for the modern andrologist

With such a wide array of computational tools to solve inference problems, andrologists and their mathematical or statistical collaborators face perhaps bewildering choices. It is tempting to criticize a method with which one is unfamiliar for its apparent complexity. Yet, many methods are quite elegant; neural computation uses nature's own best biological classifier, for example, and genetic algorithms apply rules of natural selection. Computer scientists will likely find no one single best inference engine to solve all classification problems. Rather, the modeler should choose the most appropriate computational tool based on the specific nature of a problem. If the problem can be separated into obvious components, a Markov chain may be useful. If the andrologist would like to encode a well-known clinical algorithm into the computer, the programmer may use an expert system. Once a modeler builds an inference engine, that engine is not truly useful until other andrologists use it to make inferences with their own data. Because a wide variety of computer hardware and software exists, it is a significant endeavor to translate, or "port," software designed and built on one machine to many other different computers. Fortunately, the World Wide Web offers a means by which computational tools may be made directly available to multiple users on many different systems, or "platforms." The World Wide Web refers to a standardization of information traffic on the global computer network, the Internet. The Internet is simply the linkage of many computers worldwide by computer operators who have chosen to allow other users access to their systems. Because many different types of computers exist, until recently only communication in very rudimentary form, such as text, or between select compatible machines, was available. Within the last half-decade, computer scientists and operators began to use standard means of communication between computers. Interpreters of these standard languages, such as Mosaic and Netscape, are now widely available, and they allow the casual user to access the most sophisticated multimedia aspects of computer information on a variety of different systems. Andrologists may thus use the World Wide Web to make inference engines that they have programmed available to other clinicians and researchers. For example, we programmed a World Wide Web interface to the neural networks that we trained in order to solve a number of andrology classification problems. Interested users connect to our address (at this writing http:@godot.urol.uic.edu), and they may fill out electronic forms with their own patient data, press a "predict" button, and nearly immediately view the results of our neural networks' prediction on their own computers. With the explosion in computer hardware technology, mathematics and computer science that once seemed esoteric can now be investigated on computers available to nearly all andrologists. Rapid advances in computer network technology now render a tool developed by one andrologist immediately available to many. Clearly, andrologists may expect that computational investigations in their field will be a productive ground in the near and far future.

Amiloride toxicity in the fission yeast Schizosaccharomyces pombe is released by thiamine and mutations in the thiamine-repressible gene car1

Amiloride (Am) inhibits growth in the fission yeast Schizosaccharomyces pombe. We show that the toxic effect of this drug is relieved by low concentrations of thiamine (Th) and that the pyrimidine moiety of the Th molecule is responsible for growth inhibition release. A putative membrane protein encoded by the car1 gene is the target for Am action. It is responsible for Am sensitivity and is involved in the utilization of Th and its biosynthetic precursor, 4-amino-5-hydroxymethyl-2-methylpyrimidine. Its expression is repressed by Th and is under the genetic control of the genes, thi1, tnr1, tnr2 and tnr3, which have previously been shown to be responsible for the transcriptional control of genes involved in the biosynthesis and dephosphorylation of Th.