INFLUENCES OF HISTONE STOICHIOMETRY ON THE TARGET SITE PREFERENCE OF RETROTRANSPOSONS TY1 AND TY2 IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, the target site specificity of the retrot ransposon Ty1 appears to involve the Ty integration complex recognizin g chromatin structures. To determine whether changes in chromatin stru cture affect Ty1 and Ty2 target site preference, we analyzed Ty transp osition at the CAN1 locus in mutants containing altered levels of hist one proteins. A Delta hta1-htb1 mutant with decreased levels of H2A an d H2B histone proteins showed a pattern of Ty1 and Ty2 insertions at C AN1 that was significantly different from that of both the wild-type a nd a Delta hta2-htb2 mutant, which does not have altered histone prote in levels. Altered levels of H2A and H2B proteins disrupted a dramatic orientation bias in the CAN1 promoter region. In the wild-type strain s, few Ty1 and Ty2 insertions in the promoter region were oriented opp osite to the direction of CAN1 transcription. In the Delta hta1-htb1 b ackground, however, numerous Ty1 and Ty2 insertions were in the opposi te orientation clustered within the TATA region. This altered insertio n pattern does not appear to be due to a bias caused by selecting cana vanine resistant isolates in the different HTA1-HTB1 backgrounds. Our results suggest that reduced levels of histone proteins alter Ty targe t site preference and disrupt an asymmetric Ty insertion pattern.