Threonine dehydrogenase

The region between nt 1 and nt 1071 shows 75.7% and 73.9% homology to putative alcohol dehydrogenase, zinc-containing from Acinetobacter baumannii str. AYE plasmid p2ABAYE and Salmonella enterica subsp. arizonae serovar 62:z4,z23:-- respectively. L-threonine 3-dehydrogenase (EC:1.1.1.103) is a tetrameric, zinc-binding, NAD-dependent enzyme of threonine catabolism. It catalyses the conversion of L-threonine and NAD+ to L-2-amino-3-oxobutanoate and NADH.

MobC

MobC from pRK10 is of 107 aminoacids. MobC together with MobA and MobB binds to a single cis-active site of a mobilising plasmid, the origin of transfer (oriT) region (Zhang and Meyer, 1997). The absence of MobC has several different effects on oriT DNA. Site and strand-specific nicking by MobA protein is severely reduced, accounting for the lower frequency of mobilization. The localized DNA strand separation required for this nicking is less affected, but becomes more sensitive to the level of active DNA gyrase in the cell. In addition, strand separation is not efficiently extended through the region containing the nick site. These effects suggest a model in which MobC acts as a molecular wedge for the relaxosome-induced melting of oriT DNA. The effect of MobC on strand separation may be partially complemented by the helical distortion induced by supercoiling. However, MobC extends the melted region through the nick site, thus providing the single-stranded substrate required for cleavage by MobA (Hofreuter and Haas, 2002).

MobA

Like ColE1, MobA from pRK10 is the largest of mob proteins and is composed of a polypeptide chain of 516 aminoacids; however it has been reported that only an amino-terminal segment is required for conjugal transfer (Brasch and Meyer, 1986) in ColE1. MobA is an essential component of the relaxosome, a plasmid DNA-protein complex assembled at a specific region of the plasmid called the origin of transfer (Scherzinger et al., 1992). An active relaxosome is required for an initial step in transfer, the strand-specific nicking of the plasmid DNA at a unique site within oriT (Scherzinger et al., 1992). After nicking, MobA probably becomes covalently attached to the 5' end of the cleaved strand (Scherzinger et al., 1992;Bhatachjee and Meyer, 1991). MobA is also involved in the termination of transfer (Meyer, 1989; Barlett et al., 1990). It is likely that the nicked plasmid DNA is unwound, and the linear strand transferred into the recipient cell. The strand is then ligated by the covalently attached MobA protein to reform a circular molecule (Bhatachjee and Meyer, 1991; Barlett et al., 1990).

MobB

A third protein, MobB, is also required for efficient mobilization.MobB is not required for nicking in the relaxosome ( Scherzinger et al., 1992; Zhang and Meyer, 1995), but it does stimulate this nicking in relaxosomes reconstituted in vitro from purified proteins ( Scherzinger et al., 1992).

MobD

The MobD proteins are also plasmid encoded, and are involved in the plasmids mobilization and transfer in the presence of conjugative plasmids. But its exact role in mobilization is yet to be known.