You don't necessary need to perform the RNA purification again, most of DNAse kits are suitable in terms of buffer for further applications such as cDNA synthesis. You should just inactivate the DNAse enzyme at the end of incubation by thermocycling the reaction mixture at 65-68 °C for 5 to 10 minutes according to manufacturer's datasheet. 

Furthermore you can choose to rise your samples quality for expensive downstream applications by extracting them via RNA extraction kits which have a built-in DNAse step. 

DNase application is recommended, this is to ensure that no DNA is present in your sample (could lead to erroneous conclusions if not done, purity and quality is extremely important). It would be better also to ask the company what extraction procedure they prefer.

Whether or not it's necessary might depend on what questions you're asking and how good your initial RNA extraction was. Run your sample on a Bioanalyzer or even a standard agarose gel to look for large DNA-sized bands near the top, indicating contamination. That being said I think it is a very good idea to DNAse treat your RNA prior to RNAseq no matter what you study. This way you give reviewers one less reason to doubt your results. As Paolo says, it's a fairly quick and simple protocol and you don't need to do another complete RNA purification. Trizol extractions should be fine, personally I've had success using the Ambion RNAqeous kit and Turbo DNase prior to RNAseq.

I used Qiagen kit for RNA extraction ad Fermetass for cDNA synthesis. both gave very good results. Better to treat your RNA before sequencing so that you get accurate results.

Hi Cameron, Paolo and others, thanks for your replies. Are you talking about using Ambion kit for DNase or some other brand? Heat inactivation is certainly a good method. But what about DNase buffer added in RNA, shall I submit the RNA for RNA-seq with DNAse buffer in it, as you say not to purify it? I was reading Ambion protocol, which is the same with Turbo DNase, it says:

"Some protocols suggest heating at 75°C for 5 minutes to inactivate DNase I (Huang, Fasco, and Kaminsky, 1996). We recommend a 10-minute incubation at 75°C for complete inactivation of DNase I (RNase-free) at a concentration of

0.1 U/μL. If this is the preferred method of inactivation, add EDTA to a final concentration of 5 mM before heating. If EDTA is not added, the RNA will undergo chemical scission when heated.

Note: For RNA samples that are to be used in reverse transcription reactions, the EDTA concentration in the RNA sample must be taken into account. Excess EDTA in an RNA sample may lower the free Mg2+ concentration and affect the

efficiency of reverse transcription. After heat inactivation of DNase I. It may be necessary to add additional Mg2+ for maximum reverse transcriptase activity. Alternatively, DNase I can be inactivated and removed by phenol/chloroform

extraction."

I was reading about Turbo DNase kit (not just reagent)(https://tools.lifetechnologies.com/content/sfs/manuals/cms_055740.pdf) which uses an inactivation reagent post DNase treatment and then purifies RNA just based on centrifugation (from the supernatant), looks simpler.

What shall be done, please advise, thanks?

I used the Ambion (Life Technologies) kit for RNA extraction and the Ambion Turbo DNA-free kit for DNase treatment. Yes, the standard protocol for the Turbo DNA-free kit doesn't require heat treatment, you just purify from the inactivation buffer with centrifugation. You can use the end product of the DNA-free kit as input for RNAseq etc.

I'm actually using the Invitrogen DNAse I. I perform the reaction in a final volume of 10 ul (8ul RNA + 1ul 10x buffer + 1ul DNAse I enzyme) for 15 min at RT. After the incubation I add 1ul of 25mM EDTA and heat inactivate the enzyme for 5 min at 70°C directly into the thermal cycler just before adding the cDNA synthesis mixture. Every downstream application worked well for now (qRT-PCR, pCR etc). 

I guess also every other Life Technologies product will work well in the same manner.