The RNA involved in breaking the phosphodiester bonds of hnRNA  at the junction of exons and intron regions is synthesized by the catalytic activity of 

Spliceosomes are involved in processing the hnRNA into active mRNA by a process known as splicing. Spliceosomes consisting of proteins and snRNA are involved in splicing. snRNA is formed by the catalytic activity of RNA polymerase III which also forms 5S rRNA and tRNA.