Expression of ubiquitin ligases can vary, however, the main route of regulation is phosphorylation, neddylation… This essentially depends of the pathway in which the ligase is involved. For ubiquitin ligases involved in cell cycle, for example, their expression is a function of the cell cycle progression. As far as I know, ubiquitin ligases involved in metabolism are more regulated by post-translationnal modification.

Sir i want to know at gene level how its regulated. Is it constitutively expressed or regulated by target protein or any signal?

The regulation depends of the type of ubiquitin ligase considered. Ligases involved in "housekeeping" are likely to be constituvely expressed (albeit one cannot exclude an upregulation in case of a stress to the cells), for example. I've came across a ligase where all but one subunits are constutively expressed. This other subunit is expressed upon nutrient cues, but it shows up so fast on a Western-blot that I suspect that the mRNA is already there.

For the ligases involved in some aspects of metabolism regulation, the regulation occurs mostly at the post-translational and post-transcriptional level, not at the transcriptional level. This makes sense as a cell has to react quite fast to changes in its environment. In any case, with > 500 ligases in higher eukaryotes, it's hard to give a definite answer w/o oversimplifying. If your work is about a few of these ligases, try to concentrate on finding out how these are regulated.

Thanks alot sir for such an eloborative plus specific answer... :)