PREM-2, A COPIA-TYPE RETROELEMENT IN MAIZE IS EXPRESSED PREFERENTIALLY IN EARLY MICROSPORES

We have isolated, by screening a genomic library, a retroelement from maize designated PREM-2 (pollen retroelement maize-2), which is expres sed in a tissue-specific manner. RNA transcripts of the PREM-2 family are found in the microspore but not in more mature pollen or in any of the vegetative tissues examined. The expression of PREM-2 elements in the uninucleate microspore provides an explanation for the genetic tr ansmission of genomic rearrangements caused by the transposition of re troelements. PREM-2 elements are very abundant and are estimated to co nstitute about 5% of the maize genome and could possibly have played a n important role in the determination of genome structure and in the g eneration of repetitive sequences in maize. The entire PREM-2 element is 9439 bp long. The LTRs of PREM-2 are 1307 bp in length. The interna l region between the 5' and 3' LTRs contains 6825 bp and shares homolo gy to the gag, pro, int, RT, and RNaseH regions of copia-type retroele ments. PREM-2 elements have been found in close proximity with several maize genes registered in GenBank. The presence of PREM-2 sequence in the exact 5' flanking position of three polygalacturonase genes expre ssed in pollen, has been used to examine the evolution of the polygala cturonase multigene family in maize and to estimate the time of the PR EM-2 integration event.