What is the difference between transcriptional activator and general transcription factors? Are they same or different, and do the transcriptional activators first attract general transcription factors or the RNA-polymerase also, at the same time?
Activators turn genes on - they help or promote RNA transcription of the gene. Other Transcription factors may turn genes off (prevent or reduce RNA transcription). Some transcription factors may promote or inhibit RNA transcription depending on their environment/context.
Transcription factors is such a generic term that anything can happen in practice. They are generically supposed to interact with the polymerase, but if some were found to interact by attracting other transcription factors that would not be a surprise.
I think 20+% of human genes are transcription factors (that is just a guess we haven't been able to characterize more than a few of them.
As Shigeta mentioned, transcription factors are proteins that regulate gene expression which can be either positive (activation) or negative (repression). Sometimes the same transcription factor can act as a repressor or an activator under different conditions. Transcription factors have specific target genes.
General transcription factors (GTF), however are ubiquitous proteins that are generally needed by the RNA polymerase to initiate and carry out transcription. TATA-binding protein and TAFs are some examples of GTFs.
Specific transcription factors can act via different kinds of mechanisms: as you said they can recruit RNA-polymerase (but may interact with GTFs— this has not been explicitly shown, in my knowledge). Some help in altering the chromatin structure to facilitate transcription.
General transcription factors are needed for binding of RNA polymerase and also for attachment to promotor regions of genome since most of the regions of genome is tightly bound with histones promotor region and varoius other sequences necessary for attactmant for general transcription factors might not be available.As the name suggest general transcription factors are general and common for all genes.The principle that there exit vast differentiated cells arising from a single cell sharing same genome suggest that preferential gene activation is the key to differentiated cell type,each cell has its own group of mRNA and protiens.Definitly there must be other mechanism that ensure that specific a gene is activated/deactivated in cells specific transcription factors serve this purpose as mentioned above by altering the structure of genome by chromosome remodelling enhancement of transcription or deactivation is obtained this way the transcriptional activators pave the way for prper attactment of general transcription factors and RNA polymerase to set the play for next stage recent research also suggest transcriptional activators give particular three dimensional shape to genome like protiens, to ensure gene activation by creating isolated neigbhourhoods(young et al)('rethinking transcriptional factors and gene expression' an article in MIT biology website might give deeper insight) To sum up transcriptional modulators are necessary for activation of particular genes but general transcriptional factors are necessary for all genes.