Re: can we synthesize an mRNA for a particular gene from a prokaryotic source?

If you are planning to use in vitro transcription to produce the mRNA from
your gene of interest, you will need a clone which contains the promoter
and the complete ORF, and preferably little else. So you would need the
complete sequence to generate the ideal clone. There are a couple of ways I
can imagine to do this.

1. Use the sequence of your partial clone to probe a cosmid/plasmid library
and then sequence the insert of positive clones.

2. By an arbitrary primed PCR approach. This could be messy.

The short answer, if I understand your question correctly, is that I can't
think of a way to specifically produce mRNA from your gene without knowing
the DNA sequence.

If I misunderstood, please rephrase the question and post again.