The regulation of the Sso II restriction-modification system from Shigella sonnei was studied in vivo and in vitro . In lacZ fusion experiments, Sso II methyltransferase (M. Sso II) was found to repress its own synthesis but stimulate expression of the cognate restriction endonuclease (ENase). The N-terminal 72 amino acids of M. Sso II, predicted to form a helix-turn-helix (HTH) motif, was found to be responsible for the specific DNA-binding and regulatory function of M. Sso II. Similar HTH motifs are predicted in the N-terminus of a number of 5-methylcytosine methyltransferases, particularly M. Eco RII, M.dcm and M. Msp I, of which the ability to regulate autogenously has been proposed. In vitro, the binding of M. Sso II to its target DNA was investigated using a mobility shift assay. M. Sso II forms a specific and stable complex with a 140 bp DNA fragment containing the promoter region of Sso II R-M system. The dissociation constant (Kd) was determined to be 1.5x10(-8) M. DNaseI footprinting experiments demonstrated that M. Sso II protects a 48-52 bp region immediately upstream of the M. Sso II coding sequence which includes the predicted -10 promoter sequence of M. Sso II and the -10 and -35 sequences of R. Sso II.

Integration of a DNA copy of the human immunodeficiency virus (HIV)-1 RNA genome into the human genome is an essential step in the viral replication cycle. This process is catalysed by a viral protein, integrase (IN). The first key reaction in the overall integration process is the cleavage of a dinucleotide from each 3¢-end of the viral DNA (substrate DNA), a reaction termed 3¢-end processing. In the second step, DNA strand transfer, a pair of processed DNA ends of the same viral DNA is inserted into the host cellular DNA (target DNA). The 3¢-end processing reaction requires a conserved nucleotide sequence at the viral DNA ends, but the second reaction does not absolutely require specific sequences within the host DNA. The 3¢-processing of viral DNA extremities is the first step in the integration process catalysed by human immunodeficiency virus (HIV)-1 integrase (IN). This reaction is relatively inefficient and processed DNAs are usually detected in vitro under conditions of excess enzyme. Despite such experimental conditions, steady-state Michaelis-Menten formalism is often applied to calculate characteristic equilibrium ⁄ kinetic constants of IN. We found that the amount of processed product was not significantly affected under conditions of excess DNA substrate, indicating that IN has a limited turnover for DNA cleavage. Therefore, IN works principally in a singleturnover mode and is intrinsically very slow (single-turnover rate constant ¼ 0.004 min )1 ), suggesting that IN activity is mainly limited at the chemistry step or at a stage that precedes chemistry. Moreover, fluorescence experiments showed that IN-DNA product complexes were very stable over the time-course of the reaction. Binding isotherms of IN to DNA substrate and product also indicate tight binding of IN to the reaction product. Therefore, the slow cleavage rate and limited product release prevent or greatly reduce subsequent turnover. Nevertheless, the time-course of product formation approximates to a straight line for 90 min (apparent initial velocity), but we show that this linear phase is due to the slow single-turnover rate constant and does not indicate steady-state multiple turnover. Finally, our data ruled out the possibility that there were large amounts of inactive proteins or dead-end complexes in the assay. Most of complexes initially formed were active although dramatically slow.Abbreviations HIV, human immunodeficiency virus; IN, integrase; LTR, long terminal repeat; PIC, preintegration complex; r, anisotropy; RSV, Rous sarcoma virus.

Recombinant plasmids containing fusion proteins composed of two different modules were constructed and expressed in Escherichia coli. The modules encoded the lactase LacA (LacZ) from the thermophilic bacterium Thermoanaerobacter ethanolicus and the cellulase CelD, a cellulose-binding module (CBM) from Anaerocellum thermophilum. The CelD CBM provides a spontaneous and strong sorption of the fusion proteins onto a cellulose carrier. The enzymatic activities of both the free LacA protein and LacA-CelD CBM fusion proteins immobilized onto the cellulose carrier were assessed. The LacA activity of the fusion protein was dependent upon its position with respect to the CBM. The highest level of lactase activity and stability was observed when the lactase domain was localized at its N terminus. A continuous-flow column reactor of lactase immobilized on a cellulose carrier was constructed, and its activity was assessed. The lactose hydrolysis rate for a 150 mM (5%) solution at a flow rate of 1 reactor volume per min was 75%, which is a value optimal for further whey transformation into glucose/galactose syrup.

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