I electroporated Pseudomonas aeruginosa (PA14) with a plasmid that expresses Red fluorescent protein from a lac promoter. the upstream features include a lac promoter, an operator and a little bit of the lacZ gene, thus the RFP is expressed as a fusion protein.
When I transfected e.coli K12 with this plasmid the resulting cell expressed RFP thus was bright red; however, when I transfected PA14 there was no red fluorescence. After a little research i found that PA is a lac(-) bacteria so i initially thought that there was no "lactose response" in these cells thus no activity in the lac promoter. However after some input from others I became aware that the all sigma70 promoters of which lac promoter in one, will be bound by sigma70 RNAP if the promoter is open and the RNAP will initiate transcription as long as there is no repressor.
Since my cells didn't express RFP I was wondering what could be the cause and thought perhaps if lac(-) cells contained the repressor coding lacI gene that the repressor was preventing expression...
I am stilling thinking about the possible reasons why PA14 doesn't express the RFP gene so I wanted to have a more clear understanding of what lac(-) means genetically. Does it mean there is no lac operon present? No lac anything i.e. no lacI repressor gene either?