Colonization and movement of GFP-labeled Clavibacter michiganensis subsp. michiganensis during tomato infection

The vascular pathogen Clavibacter michiganensis subsp. michiganensis is responsible for bacterial wilt and canker of tomato. Pathogenicity of this bacterium is dependent on plasmid-borne virulence factors and serine proteases located on the chromosomal chp/tomA pathogenicity island (PAI). In this study, colonization patterns and movement of C. michiganensis subsp. michiganensis during tomato infection was examined using a green fluorescent protein (GFP)-labeled strain. A plasmid expressing GFP in C. michiganensis subsp. michiganensis was constructed and found to be stable in planta for at least 1 month. Confocal laser-scanning microscopy (CLSM) of inoculated stems showed that the pathogen extensively colonizes the lumen of xylem vessels and preferentially attaches to spiral secondary wall thickening of the protoxylem. Acropetal movement of the wild-type strain C. michiganensis subsp. michiganensis NCPPB382 (Cmm382) in tomato resulted in an extensive systemic colonization of the whole plant reaching the apical region after 15 days, whereas Cmm100 (lacking the plasmids pCM1 and pCM2) or Cmm27 (lacking the chp/tomA PAI) remained confined to the area surrounding of the inoculation site. Cmm382 formed biofilm-like structures composed of large bacterial aggregates on the interior of xylem walls as observed by CLSM and scanning electron microscopy. These findings suggest that virulence factors located on the chp/tomA PAI or the plasmids are required for effective movement of the pathogen in tomato and for the formation of cellular aggregates.

Plant adaptation to dynamically changing environment: the shade avoidance response

The success of competitive interactions between plants determines the chance of survival of individuals and eventually of whole plant species. Shade-tolerant plants have adapted their photosynthesis to function optimally under low-light conditions. These plants are therefore capable of long-term survival under a canopy shade. In contrast, shade-avoiding plants adapt their growth to perceive maximum sunlight and therefore rapidly dominate gaps in a canopy. Daylight contains roughly equal proportions of red and far-red light, but within vegetation that ratio is lowered as a result of red absorption by photosynthetic pigments. This light quality change is perceived through the phytochrome system as an unambiguous signal of the proximity of neighbors resulting in a suite of developmental responses (termed the shade avoidance response) that, when successful, result in the overgrowth of those neighbors. Shoot elongation induced by low red/far-red light may confer high relative fitness in natural dense communities. However, since elongation is often achieved at the expense of leaf and root growth, shade avoidance may lead to reduction in crop plant productivity. Over the past decade, major progresses have been achieved in the understanding of the molecular basis of shade avoidance. However, uncovering the mechanisms underpinning plant response and adaptation to changes in the ratio of red to far-red light is key to design new strategies to precise modulate shade avoidance in time and space without impairing the overall crop ability to compete for light.

Osteosynthesis systems in fragility fracture

Atraumatic fractures, more commonly known as "fragility fractures", have as their basis the decreased bone strength due to osteoporosis. The major technical problem is the difficulty in obtaining secure fixation in osteoporotic bone, because the pull-out strength of implant is significantly reduced. The high rate of complications has encouraged extensive research into the development of implant which can improve the bone-implant interface by preventing high stress and distributing the forces transmitted to bone in a load-sharing, rather than load-bearing way.

The incidence of hip, forearm, humeral, ankle, and vertebral fragility fractures in Italy: results from a 3-year multicenter study

Introduction

We aimed to assess the incidence and hospitalization rate of hip and "minor" fragility fractures in the Italian population.

Methods

We carried out a 3-year survey at 10 major Italian emergency departments to evaluate the hospitalization rate of hip, forearm, humeral, ankle, and vertebral fragility fractures in people 45 years or older between 2004 and 2006, both men and women. These data were compared with those recorded in the national hospitalizations database (SDO) to assess the overall incidence of fragility fractures occurring at hip and other sites, including also those events not resulting in hospital admissions.

Results

We observed 29,017 fractures across 3 years, with hospitalization rates of 93.0% for hip fractures, 36.3% for humeral fractures, 31.3% for ankle fractures, 22.6% for forearm/wrist fractures, and 27.6% for clinical vertebral fractures. According to the analyses performed with the Italian hospitalization database in year 2006, we estimated an annual incidence of 87,000 hip, 48,000 humeral, 36,000 ankle, 85,000 wrist, and 155,000 vertebral fragility fractures in people aged 45 years or older (thus resulting in almost 410,000 new fractures per year). Clinical vertebral fractures were recorded in 47,000 events per year.

Conclusions

The burden of fragility fractures in the Italian population is very high and calls for effective preventive strategies.

Sequential expression of bacterial virulence and plant defense genes during infection of tomato with Clavibacter michiganensis subsp. michiganensis

The molecular interactions between Clavibacter michiganensis subsp. michiganensis and tomato plant were studied by following the expression of bacterial virulence and host-defense genes during early stages of infection. The C. michiganensis subsp. michiganensis genes included the plasmid-borne cellulase (celA) and the serine protease (pat-1), and the serine proteases chpC and ppaA, residing on the chp/tomA pathogenicity island (PAI). Gene expression was measured following tomato inoculation with Cmm382 (wild type), Cmm100 (lacking the plasmids pCM1 and pCM2), and Cmm27 (lacking the PAI). Transcriptional analysis revealed that celA and pat-1 were significantly induced in Cmm382 at initial 12 to 72 h, whereas chpC and ppaA were highly expressed only 96 h after inoculation. Interdependence between the expression of chromosomal and of plasmid-located genes was revealed: expression of celA and pat-1 was substantially reduced in the absence of the chp/tomA PAI, whereas chpC and ppaA expressions were reduced in the absence of the virulence plasmids. Transcription of chromosomal genes involved in cell wall degradation (i.e., pelA1, celB, xysA, and xysB), was also induced at early stages of infection. Expression of the host-defense genes, chitinase class II and pathogenesis-related protein-5 isoform was induced in the absence of the PAI at early stages of infection, suggesting that PAI-located genes are involved in suppression of tomato basal defenses.

Bilateral total hip arthroplasty in subjects with multiple epiphyseal dysplasia

Multiple epiphyseal dysplasia (MED) is a rare autosomal dominant skeletal dysplasia caused by abnormalities in the gene(s) coding for type IX collagen and cartilage oligomeric matrix protein. Clinically, the disease is characterized by abnormalities in the epiphyseal cartilage of multiple joints. Orthopedic manifestations include pain and restricted mobility. The disorder, which has a predilection for the femoral head and acetabulum, usually presents from the third to the fourth decade with secondary hip osteoarthritis. We report on 7 subjects affected by multiple epiphyseal dysplasia, which presented hip osteoarthritis in their third decade of life and underwent staged bilateral total hip arthroplasty. All patients experienced pain relief and improved quality of life after surgical treatment.

Glioblastoma multiforme of the conus medullaris in a child: description of a case and literature review

We describe a case of glioblastoma multiforme of the conus medullaris in a child. MR imaging showed at the T12-L1 level an intramedullary mass with signal alteration, with only two nodules of contrast enhancement. The finding was consistent with astrocytoma. Pathologic evaluation was consistent with glioblastoma multiforme. Nine months after the surgical treatment, the patient showed leptomeningeal recurrence. This case appears to be unusual for both the hysto-type and the nonspecific MR imaging features.

Riluzole restores motor activity in rats with post-traumatic peripheral neuropathy

Riluzole is a presynaptic inhibitor of glutamate release with neuroprotective properties. In order to evaluate the effects of riluzole on motor activity in post-traumatic peripheral neuropathy (PTPN), the sciatic nerve of Wistar male rats was exposed monolaterally and subjected to crushing for one min by a surgical forceps. Animals received an intraperitoneal treatment with riluzole (2, 4 or 8 mg/kg per day), diclofenac (5, 10 or 20 mg/kg) or with vehicle for 3 days. Motor activity and coordination was evaluated in a circular open field and in the rotorod test. The treatment with riluzole stimulated ambulation in PTPN rats and improved their motor performance and coordination. The effect of treatment with riluzole on locomotor activity was greater than that of treatment with diclofenac and was dose-dependent. Furthermore, in contrast to vehicle- and diclofenac-treated rats, animals treated with riluzole showed a long-lasting improvement of locomotor activity as it was assessed 7 days after the end of treatment. These findings suggest that riluzole may improve motor performance in PTPN, and this does not depend on its antinociceptive activity. Its neuroprotective properties are possibly involved in this effect.

Heterotopic ossification after total hip replacement and the HLA system in the Sicilian population

Heterotopic ossification (HO) is a frequent complication following total hip arthroplasty (THA). At present, the etiology HO is unknown, however, genetic predisposition may be a cause of HO in individuals in whom no risk factors can be detected. The goal of this study was to investigate the HLA system, searching for any correlation with the presence of HO after THA. Thirty-five patients of Sicilian origin were operated on between January 1997 and January 1999 for cementless THA under regional anesthesia. The entire series was divided into three groups and all underwent histocompatibility typing. Group I was made up of 10 patients who presented with HO Brooker grades 1 and 2 after THA; group 2 comprised 7 patients affected by grades 3 and 4 HO after THA; and group 3 was made up of 18 subjects who presented with one or more preoperative risk factors for developing peri-prosthetic HO before undergoing THA. No positivity for HLA-B27 antigen was observed, but there was as an increase in HLA-B18 (with respect to that in the Sicilian population) in patients with HO following THA. The main conclusion from the study is that there is a strong correlation between the presence of the antigens HLA-A2 and HLA-B18 in patients with HO grades 3 and 4.

Innate immunity in plants

Studies of receptors and signal-transduction components that play a role in plant disease resistance have revealed remarkable similarities with innate immunity pathways in insects and mammals. In plants, specific receptors encoded by disease-resistance genes interact with products of microbial effector genes to activate defence responses. Resistance proteins have been found to have motifs in common with components of immune response pathways in mammals and invertebrates, and to rely on similar downstream signalling components. In the future, the sharing of ideas among plant and animal biologists is likely to broaden our understanding of defence responses in diverse organisms.

Signal recognition and transduction mediated by the tomato Pto kinase: a paradigm of innate immunity in plants

Plant disease resistance is the result of an innate host defense mechanism, which relies on the ability of the plant to recognize pathogen invasion and to efficiently mount defense responses. In tomato, resistance to the pathogen Pseudomonas syringae is mediated by the specific interaction between the plant serine/threonine kinase Pto and the bacterial protein AvrPto. This article reviews molecular and biochemical properties that confer to Pto the capability to function as an intracellular receptor and to activate a signaling cascade leading to the induction of defense responses.

Subjects positive for hepatitis C virus RNA with normal aminotransferase levels, a "trompe l'oeil" clinical picture?

BACKGROUND

Quite often subjects affected by chronic hepatitis C virus infection have no clinical signs of liver disease and serum aminotransferase values never go beyond the upper limit of normal. Yet these subjects, defined "asymptomatic HCV carriers", often have active viral replication and various degrees of histological damage.

AIMS

To verify, in a population of antibody to hepatitis C virus carriers, if normal serum aminotransferase values in hepatitis C virus-RNA positive differed considerably from those in hepatitis C virus-RNA negative subjects.

SUBJECTS/METHODS

We followed 24 anti-hepatitis C virus-positive subjects (15 hepatitis C virus-RNA positive and 9 negative) by measuring alanine and aspartate aminotransferase levels at 3-month intervals for a median of 40 months (range 6-77).

RESULTS

Determinations resulted repeatedly and rigorously within the normal range in all participants. Alanine aminotransferase values were higher in hepatitis C virus-RNA positives than in negatives (mean +/- SD: 0.609+/-0.172 vs 0.434+/-0.153 times the upper limit of normal; p<O.001). Aspartate aminotransferase values in both groups reflected the same consistent difference (mean +/- SD: 0.652+/-0.170 vs 0.528+/-0.151 times the upper limit of normal, p<0.001).

CONCLUSIONS

Our findings demonstrate that biochemical activity exists in asymptomatic hepatitis C virus-RNA-positive carriers, but it is so slight that serum aminotransferase levels do not go beyond the upper limit of normal. With the prevailing range of normality, serum aminotransferase determination is a helpful but not reliable marker of disease activity in chronic hepatitis C virus infection. The normal range for aminotransferases needs to be redefined in order to set up a more accurate diagnostic profile of subjects with chronic hepatitis C virus infection.

Thr38 and Ser198 are Pto autophosphorylation sites required for the AvrPto-Pto-mediated hypersensitive response

The tomato Pto kinase confers resistance to Pseudomonas syringae pv. tomato expressing the AvrPto protein. To elucidate the role of Pto autophosphorylation in disease resistance, eight sites autophosphorylated by Pto in vitro were identified by a combination of HPLC purification of tryptic phosphopeptides, MALDI-TOF/MS analysis and Edman degradation. Mutational analysis of the autophosphorylation sites revealed that Pto residues Thr38 and Ser198 are required for AvrPto-Pto- mediated elicitation of a hypersensitive response in the plant. Thr38, which is the main Pto autophosporylation site and is located outside the kinase catalytic domain, was also required for Pto kinase activity and its physical interaction with AvrPto, the Pti1 kinase and the transcription factor Pti4. Ser198, located in the Pto activation domain, was dispensable for kinase activity and for interaction with AvrPto. However, a mutation at this site resulted in altered Pto interactions with the Pti1 kinase and the Pto interactors of unknown function Pti3 and Pti10. These results suggest that autophosphorylation events at Pto Thr38 and Ser198 are required for signal transduction by Pto and participate in distinct molecular mechanisms.

The major site of the pti1 kinase phosphorylated by the pto kinase is located in the activation domain and is required for pto-pti1 physical interaction

The Pto and Pti1 serine/threonine protein kinases are key components of the signaling pathway leading to speck disease resistance in tomato. The two kinases physically interact in the yeast two-hybrid system, and Pto specifically phosphorylates Pti1 in vitro. In this study, we identified and characterized the major Pti1 site phosphorylated by Pto. Pto was expressed in Escherichia coli as a maltose-binding fusion protein (MBP-Pto), and used to phosphorylate in vitro a kinase deficient Pti1 protein fused to glutathione S-transferase (GST-Pti1[K96N]). The major phosphopeptide derived from trypsin digestion of phosphorylated GST-Pti1(K96N) was partially purified by reverse-phase HPLC and analyzed by matrix assisted laser desorption/ionization mass spectrometry. Its mass corresponded to phosphopeptide LHSTR, which lies in the Pti1 kinase activation domain at amino acid position 230-234. By phosphoamino acid analysis, Thr233 was determined to be the phosphorylation site of peptide LHSTR. Mutations of Thr233 reduced dramatically Pti1 phosphorylation by MBP-Pto and Pti1 autophosphorylation, providing evidence that the same Pti1 site is involved in the two reactions. Moreover, phosphorylation of Thr233 appeared to be required for Pto-Pti1 physical interaction, as a mutation of this site to alanine, but not to aspartate, abolished the interaction between Pto and Pti1 in the yeast two-hybrid system.

The ethylene-inducible PK12 kinase mediates the phosphorylation of SR splicing factors

The tobacco PK12 is induced by the plant hormone ethylene and is a member of the LAMMER family of protein kinases. Members of this family contain in their C-terminus a unique 'EHLAMMERI/VLGPLP' motif of unknown function, and are related to cyclin- and mitogen-activated protein (MAP)-dependent kinases. The animal members of this class play a role in differentiation. They phosphorylate and physically interact with serine/arginine-rich (SR) splicing factors in vivo to alter their activity and the splicing of target mRNAs. SR proteins have been recently described in plants. The capability of PK12 LAMMER kinase to bind and phosphorylate SR proteins was tested in vitro by kinase and binding assays. The tobacco PK12 phosphorylated both animal and plant SR proteins and specifically interacted with the plant splicing factor atSRp34/SR1. In addition, by site-directed mutagenesis, the LAMMER motif was found to be required for PK12 kinase activity but was not necessary for substrate binding. Consistent with a role in phosphorylation of splicing factors, PK12 was found to localize to the nucleus when transiently over-expressed in suspension cells.

Shade avoidance responses are mediated by the ATHB-2 HD-zip protein, a negative regulator of gene expression

The ATHB-2 gene encoding an homeodomain-leucine zipper protein is rapidly and strongly induced by changes in the ratio of red to far-red light which naturally occur during the daytime under the canopy and induce in many plants the shade avoidance response. Here, we show that elevated ATHB-2 levels inhibit cotyledon expansion by restricting cell elongation in the cotyledon-length and -width direction. We also show that elevated ATHB-2 levels enhance longitudinal cell expansion in the hypocotyl. Interestingly, we found that ATHB-2-induced, as well as shade-induced, elongation of the hypocotyl is dependent on the auxin transport system. In the root and hypocotyl, elevated ATHB-2 levels also inhibit specific cell proliferation such as secondary growth of the vascular system and lateral root formation. Consistent with the key role of auxin in these processes, we found that auxin is able to rescue the ATHB-2 lateral root phenotype. We also show that reduced levels of ATHB-2 result in reciprocal phenotypes. Moreover, we demonstrate that ATHB-2 functions as a negative regulator of gene expression in a transient assay. Remarkably, the expression in transgenic plants of a derivative of ATHB-2 with the same DNA binding specificity but opposite regulatory properties results in a shift in the orientation of hypocotyl cell expansion toward radial expansion, and in an increase in hypocotyl secondary cell proliferation. A model of ATHB-2 function in the regulation of shade-induced growth responses is proposed.

Clinical outcome of chronic hepatitis C in patients treated with interferon: comparison between responders and non-responders

AIM

To evaluate the prognosis of chronic hepatitis C in relation to interferon therapy response and the persistence of therapeutic benefits.

PATIENTS/METHODS

We studied the clinical outcome of 191 patients with chronic infection (152 chronic hepatitis C and 39 cirrhosis) treated with recombinant alpha-interferon (3-6 MU on alternate days for 1 year) during a mean period of 47 months (range 22.5-73.8). Control tests were done at 6-month intervals. HCV RNA was determined pre- and post-treatment in all participants, but continued yearly in long-term responders. The appearance of cirrhosis was estimated using a non-invasive method that utilizes a model based on clinical, instrumental and biochemical variables. Ascites, encephalopathy, haemorrhage, hepatocellular carcinoma, and death were considered liver-disease-related events.

RESULTS

A total of 39 patients were long-term responders, 36 relapsers, and 116 non-responders; 92% of long-term responders cleared HCV RNA and remained negative throughout the study period. The 3 HCV-RNA-positive long-term responders continued being so. No biochemical relapse was observed in long-term responders regardless of virological status. New cirrhosis was observed in 3/30 relapsers, in 9/85 non-responders, and in no long-term responders. Overall, 9 episodes of severe events occurred in 20% of cirrhotics and in 0.6% of chronic hepatitis, all non-responders.

CONCLUSIONS

Long-term response interrupts the progression to cirrhosis and reduces the incidence of severe complications. Multivariate analysis revealed that "baseline diagnosis of cirrhosis" was the only independent factor predictive of an unfavourable outcome of chronic HCV-related liver disease.

The Arabidopsis Athb-8, -9 and -14 genes are members of a small gene family coding for highly related HD-ZIP proteins

We report the isolation and characterization of two Arabidopsis homeobox genes highly related to the Athb-8 gene. The full-length cDNAs encode proteins of 841 and 852 amino acids which we have designated Athb-9 and -14, respectively. Athb-8, -9 and -14 are members of a small family of HD-Zip proteins (HD-ZIP III) characterized by a HD-Zip motif confined to the N-terminus of the polypeptide. The spatial organization of the HD-Zip domain of Athb-8, -9 and -14 is different from that of the Athb-1 (a member of the HD-ZIP I family) and Athb-2 (a member of the HD-ZIP II family) HD-Zip domains. DNA binding analysis performed with random-sequence DNA templates showed that the Athb-9 HD-Zip (HD-Zip-9) domain, but not the Athb-9 HD alone, binds to DNA. The HD-Zip-9 domain recognizes a 11 bp pseudopalindromic sequence (GTAAT(G/C)ATTAC), as determined by selecting high-affinity binding sites from random-sequence DNA. Moreover, gel retardation assays demonstrated that the HD-Zip-9 domain binds to DNA as a dimer. These data support the notion that the HD-ZIP III domain interacts with DNA recognition elements in a fashion similar to the HD-ZIP I and II domains.

Recognition specificity for the bacterial avirulence protein AvrPto is determined by Thr-204 in the activation loop of the tomato Pto kinase

The Pto kinase confers resistance in tomato to P. syringae pv. tomato strains expressing the AvrPto protein. Physical interaction of the Pto kinase and AvrPto protein in the plant cell initiates host defense responses. The recognition event between these two proteins is very specific; AvrPto does not interact with other closely related kinases, including the Fen kinase, which shares 80% amino acid identity with Pto. By using Pto-Fen chimeric proteins and site-directed mutagenesis, we found that Thr-204 is required for Pto interaction with AvrPto in a yeast two-hybrid system and for recognition specificity in a tobacco leaf transient assay. Substitution of Thr-204 into the Fen kinase allowed that kinase to interact with AvrPto and to confer an AvrPto-specific defense response in tobacco leaves. Thus, simple mutations appear capable of giving rise to new resistance gene specificities.

Biochemical properties of two protein kinases involved in disease resistance signaling in tomato

In tomato plants, resistance to bacterial speck disease is mediated by a phosphorylation cascade, which is triggered by the specific recognition between the plant serine/threonine protein kinase Pto and the bacterial AvrPto protein. In the present study, we investigated in vitro biochemical properties of Pto, which appears to function as an intracellular receptor for the AvrPto signal molecule. Pto and its downstream effector Pti1, which is also a serine/threonine protein kinase, were expressed in Escherichia coli as maltose-binding protein and glutathione S-transferase fusion proteins, respectively. The two kinases each autophosphorylated at multiple sites as determined by phosphopeptide mapping. In addition, Pto and Pti1 autophosphorylation occurred via an intramolecular mechanism, as their specific activity was not affected by their molar concentration in the assay. Moreover, an active glutathione S-transferase-Pto fusion failed to phosphorylate an inactive maltose-binding protein-Pto(K69Q) fusion excluding an intermolecular mechanism of phosphorylation for Pto. Pti1 phosphorylation by Pto was also characterized and found to occur with a Km of 4.1 microM at sites similar to those autophosphorylated by Pti1. Pto and the product of the recessive allele pto phosphorylated Pti1 at similar sites, as observed by phosphopeptide mapping. This suggests that the inability of the kinase pto to confer resistance to bacterial speck disease in tomato is not caused by altered recognition specificity for Pti1 phosphorylation sites.

Chronic hepatitis C long-term responders to human leukocyte interferon-alpha therapy: persistence of a sustained biochemical and virological response during 5 years of surveillance

OBJECTIVES

To define the biochemical and virological course and IgM response to HCV-core protein in long-term responders (LTRs) during a long surveillance (5 years).

DESIGN

From 1989 to 1991, 98 patients (pts) with biopsy-proven chronic hepatitis C were enrolled into this study. These pts underwent human leukocyte interferon-alpha (LE-IFN alpha) therapy at the prolonged schedule (3 MU thrice weekly for 1 year).

METHODS

Serum alanine-aminotransferases (ALTs) were assessed monthly during and until 1 year after treatment, then every 3 months during the observation period. Qualitative and quantitative HCV RNA and HCV IgM were measured in all pts on baseline samples and in LTRs also after treatment and every following year.

RESULTS

Based on serum ALT course, the pts were defined as: LTRs (14 pts), if their serum ALT levels returned to the normal range during therapy and remained so for at least 1 year afterwards; responders with relapse (RRs, 20 pts), if their serum ALT levels returned to the normal range during therapy but increased after ending treatment; and non-responders (NRs, 64 pts), if their serum ALT levels remained abnormal throughout therapy. No significant differences were seen regarding IgM anti-HCV positivity and serum ALT levels among the three groups. LTRs (12 HCV-RNA negative and two HCV-RNA positive at the end of treatment) maintained their virological status and not one of them experienced an elevation of serum ALT levels throughout the surveillance.

CONCLUSION

Patients affected by chronic hepatitis C and treated with interferon, but who did not experience a biochemical or virological relapse within the first year of follow-up would not relapse later on; thus, we are able to conclude that these subjects made a complete recovery.

DNA-binding specificity of the homeodomain-leucine zipper domain

Homeodomain-leucine zipper (HD-Zip) proteins are putative transcription factors identified only in plants. The study of the DNA-binding properties of the ATHB-1 and -2 HD-Zip (HD-Zip-1 and -2) domains showed that they interact with DNA as homodimers and recognize two distinct 9 bp pseudopalindromic sequences, CAAT(A/T)ATTG (BS-1) and CAAT(G/C)ATTG (BS-2), respectively, as determined by selecting high-affinity binding sites from random-sequence DNA. Here, we report a mutational analysis of the HD-Zip-2 domain. We determined that conserved amino acid residues of helix 3, Val47 and Asn51, and Arg55 are essential for the DNA-binding activity of the HD-Zip-2 domain. We demonstrated that the preferential recognition of a G/C base-pair at the central position by the HD-Zip-2 domain is abolished either by the replacement of Arg55 with lysine or by the substitution of Glu46 and Thr56 with the corresponding residues of the HD-Zip-1 domain (alanine and tryptophan, respectively). In contrast, substitution of Arg55 with lysine in the HD-Zip-1 domain significantly reduced DNA-binding activity without changing the specificity of recognition. Finally, we determined that differences in residues outside helix 3 further contribute to the DNA-binding specificity of the HD-Zip domain. Taken together, the data strongly suggest that the preferential recognition of BS-2 and -1 by the HD-Zip-2 and -1 domains, respectively, may be attributable to a distinct orientation of the side-chain of Arg55 in these two domains.